Method and device for purifying exhaust gas of engine

ABSTRACT

An engine (1) has first and second cylinder groups (1a) and (1b). The first cylinder group (1a) is connected to a three way (TW) catalyst (8a). The second group (1b) and the TW catalyst (8a) are connected, via an interconnecting duct (13) to an NH 3  adsorbing and oxidizing (NH 3  -AO) catalyst (14a). The first group (1a) performs the rich operation, and the second group (1b) performs the lean operation. In the TW catalyst (8a), NO x  exhausted from the first group (1a) is converted to NH 3 , and the NH 3  reduces the NO x  exhausted from the second group (1b) in the NH 3  -AO catalyst (14a). A NO x  occluding and reducing (NO x  -OR) catalyst (11a) is arranged in the exhaust passage between the second group (1b) and the interconnecting duct (13), to thereby suppress the NO x  amount flowing into the NH 3  -AO catalyst (14a).

TECHNICAL FIELD

The present invention relates to a method and a device for purifying an exhaust gas of an engine.

BACKGROUND ART

If an air-fuel ratio of an air-fuel mixture in a combustion chamber of an internal combustion engine is referred as an engine air-fuel ratio, and if a ratio of the total amount of air fed into the intake passage, the combustion chamber, and the exhaust passage upstream of a certain position in the exhaust passage to the total amount of fuel fed into the intake passage, the combustion chamber, and the exhaust passage upstream of the above-mentioned position is referred to as an exhaust gas air-fuel ratio of the exhaust gas flowing through the certain position, the Japanese Unexamined Patent Publication No. 4-365920 discloses an exhaust gas purifying device for an internal combustion engine with multi-cylinders, the engine having first and second cylinder groups, in which the device is provided with: an engine operation control device to make each cylinder of the first cylinder group a rich engine operation in which the engine air-fuel ratio is rich, and to make each cylinder of the second cylinder group a lean engine operation in which the engine air-fuel ratio is lean; a first exhaust passage connected to each cylinder of the first cylinder group; a second exhaust passage connected to each cylinder of the second cylinder group and different from the first exhaust passage; an NH₃ synthesizing catalyst arranged in the first exhaust passage for synthesizing ammonia NH₃ from at least a part of NO_(x) in the inflowing exhaust gas; an interconnecting passage interconnecting the first exhaust passage downstream of the NH₃ synthesizing catalyst and the second exhaust passage to each other; and an exhaust gas purifying catalyst arranged in the interconnecting passage to reduce NO_(x) from the second exhaust passage by NH₃ from the first exhaust passage.

In the above engine, the fuel consumption rate is reduced by increasing the numbers of the cylinders of the second cylinder group in which the lean engine operation is performed. However, if the numbers of the cylinders of the first group are decreased and the numbers of the cylinders of the second group are increased, the NH₃ amount flowing into the exhaust gas purifying catalyst decreases and the NO_(x) amount flowing into the catalyst increases. As a result, the NO_(x) amount flowing into the catalyst may be excessive with respect to the NH₃ amount, and thus NO_(x) may be emitted from the catalyst without being reduced sufficiently.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method and a device for purifying an exhaust gas of an engine which can suppress the amount of NO_(x) flowing into an exhaust gas purifying catalyst with respect to that of NH₃, to thereby purify the exhaust gas sufficiently.

According to one aspect of the present invention, there is provided a method for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, the method comprising: making an exhaust gas air-fuel ratio of the exhaust gas of the first cylinder group rich, and introducing the exhaust gas to an NH₃ synthesizing catalyst to synthesize NH₃, the NH₃ synthesizing catalyst synthesizing NH₃ from at least a part of NO_(x) in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; introducing the exhaust gas of the first cylinder group including NH₃ and the exhaust gas of the second cylinder group including NO_(x) together to an exhaust gas purifying catalyst; and controlling an amount of NO_(x) included in the exhaust gas of the second cylinder group and to be introduced to the exhaust gas purifying catalyst to prevent the NO_(x) amount from being larger than a NO_(x) amount which can be reduced by the NH₃ included in the exhaust gas of the first cylinder group and to be introduced to the exhaust gas purifying catalyst, wherein, on the exhaust gas purifying catalyst, the inflowing NO_(x) is reduced by the inflowing NH₃.

According to another aspect of the present invention, there is provided a device for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, and first and second exhaust passage connected to the first and the second cylinder groups, respectively, the device comprising: an NH₃ synthesizing catalyst arranged in the first exhaust passage, the NH₃ synthesizing catalyst synthesizing NH₃ from at least a part of NO_(x) in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; an interconnecting exhaust passage interconnecting the first passage downstream of the NH₃ synthesizing catalyst and the second exhaust passage; an exhaust gas purifying catalyst arranged in the interconnecting passage for reducing the inflowing NO_(x) by the inflowing NH₃ ; first exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst; means for controlling the first exhaust gas air-fuel ratio control means to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst rich to synthesize NH₃ ; and NO_(x) amount control means for controlling an amount of NO_(x) flowing from the second exhaust passage into the exhaust gas purifying catalyst to prevent the NO_(x) amount from being larger than a NO_(x) amount which can be reduced by the NH₃ flowing from the first exhaust passage into the exhaust gas purifying catalyst, wherein, on the exhaust gas purifying catalyst, the inflowing NO_(x) is reduced by the inflowing NH₃.

According to another aspect of the present invention, there is provided a method for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, the method comprising: making the exhaust gas air-fuel ratio of the exhaust gas of the first cylinder group rich, and introducing the exhaust gas to an NH₃ synthesizing catalyst to synthesize NH₃, to form the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich, the NH₃ synthesizing catalyst synthesizing NH₃ from at least a part of NO_(x) in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; making the exhaust gas air-fuel ratio of the exhaust gas of the second cylinder group lean, to form the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean; performing a first introducing condition where the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich is introduced to an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst and the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean is introduced to a NO_(x) occluding and reducing (NO_(x) -OR) catalyst, the NH₃ -AO catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and desorbing the adsorbed NH₃ therefrom and oxidizing the NH₃ when the NH₃ concentration in the inflowing exhaust gas becomes lower, the NO_(x) -OR catalyst occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; performing a second introducing condition where the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich is introduced to the NO_(x) -OR catalyst and the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean is introduced to the NH₃ -AO catalyst; and performing the first and the second introducing conditions alternately and repeatedly.

According to further another aspect of the present invention, there is provided a device for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, and first and second exhaust passage connected to the first and the second cylinder groups, respectively, the device comprising: an NH₃ synthesizing catalyst arranged in the first exhaust passage, the NH₃ synthesizing catalyst synthesizing NH₃ from at least a part of NO_(x) in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst selectively connected to one of the first exhaust passage downstream of the NH₃ synthesizing catalyst and the second exhaust passage, the NH₃ -AO catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and desorbing the adsorbed NH₃ therefrom and oxidizing the NH₃ when the NH₃ concentration in the inflowing exhaust gas becomes lower; a NO_(x) occluding and reducing (NO_(x) -OR) catalyst selectively connected to one of the first exhaust passage downstream of the NH₃ synthesizing catalyst and the second exhaust passage, the NO_(x) -OR catalyst occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; first exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst; second exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing through the second exhaust passage; means for controlling the first exhaust gas air-fuel ratio control means to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst rich to synthesize NH₃ ; means for controlling the second exhaust gas air-fuel ratio control means to make the exhaust gas air-fuel ratio of the exhaust gas flowing through the second exhaust passage lean; first connecting condition performing means for performing a first connecting condition where the first exhaust passage downstream of the NH₃ synthesizing catalyst is connected to the NH₃ -AO catalyst and the second exhaust passage is connected to the NO_(x) -OR catalyst; second connecting condition performing means for performing a second connecting condition where the first exhaust passage downstream of the NH₃ synthesizing catalyst is connected to the NO_(x) -OR catalyst and the second exhaust passage is connected to the NH₃ -AO catalyst; and connecting condition control means for controlling the first and the second connecting condition performing means to perform the first and the second connecting conditions alternately and repeatedly.

According to further another aspect of the present invention, there is provided a method for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, the method comprising: introducing the exhaust gas of the first cylinder group to a first NH₃ synthesizing catalyst and an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst, in turn, the NH₃ synthesizing catalyst synthesizing NH₃ from at least a part of NO_(x) in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and passing NO_(x) in the inflowing exhaust gas therethrough when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and the NH₃ -AO catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and desorbing the adsorbed NH₃ therefrom and oxidizing the NH₃ when the NH₃ concentration in the inflowing exhaust gas becomes lower; introducing the exhaust gas of the second cylinder group to a second NH₃ synthesizing catalyst and a NO_(x) occluding and reducing (NO_(x) -OR) catalyst, in turn, the NO_(x) -OR catalyst occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; performing a first exhaust gas air-fuel ratio condition where the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst is made rich, and that of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst is made lean; performing a second exhaust gas air-fuel ratio condition where the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst is made lean, and that of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst is made rich; and performing the first and the second exhaust gas air-fuel ratio conditions alternately and repeatedly.

According to another aspect of the present invention, there is provided a device for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, and first and second exhaust passage connected to the first and the second cylinder groups, respectively, the device comprising: a first NH₃ synthesizing catalyst arranged in the first exhaust passage and a second NH₃ synthesizing catalyst arranged in the second exhaust passage, each NH₃ synthesizing catalyst synthesizing NH₃ from at least a part of NO_(x) in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and passing NO_(x) in the inflowing exhaust gas therethrough when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean; an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst arranged in the first exhaust passage downstream of the first NH₃ synthesizing catalyst, the NH₃ -AO catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and desorbing the adsorbed NH₃ therefrom and oxidizing the NH₃ when the NH₃ concentration in the inflowing exhaust gas becomes lower; a NO_(x) occluding and reducing (NO_(x) -OR) catalyst arranged in the second exhaust passage downstream of the second NH₃ synthesizing catalyst, the NO_(x) -OR catalyst occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; a first exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst; a second exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst; first exhaust gas air-fuel ratio condition performing means for controlling the first and the second exhaust gas air-fuel ratio control means to perform a first exhaust gas air-fuel ratio condition where the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst is made rich, and that of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst is made lean; second exhaust gas air-fuel ratio condition performing means for controlling the first and the second exhaust gas air-fuel ratio control means to perform a second exhaust gas air-fuel ratio condition where the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst is made lean, and that of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst is made rich; and exhaust gas air-fuel ratio condition control means for controlling the first and the second exhaust gas air-fuel ratio condition performing means to perform the first and the second exhaust gas air-fuel ratio conditions alternately and repeatedly.

The present invention may be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general view of an internal combustion engine;

FIG. 2 illustrates a characteristic of a three-way catalyst;

FIGS. 3 and 4 schematically illustrate a method for purifying the exhaust gas according to the embodiment shown in FIG. 1;

FIG. 5 is a time chart for explaining the exhaust gas purifying method according to the embodiment shown in FIG. 1;

FIGS. 6A and 6B are diagrams illustrating a NO_(x) amount exhausted from the second cylinder group per unit time;

FIGS. 7A and 7B are diagrams illustrating a NO_(x) amount passing through the NO_(x) -OR catalyst per unit time;

FIGS. 8A and 8B are diagrams illustrating a NO_(x) amount released from the NO_(x) -OR catalyst per unit time;

FIG. 9 is a diagram illustrating a temperature of the exhaust gas flowing into the NO_(x) -OR catalyst;

FIG. 10 is a flow chart for executing an operation change control;

FIG. 11 is a flow chart for calculating a fuel injection time;

FIG. 12 is a time chart for explaining the exhaust gas purifying method according to another embodiment;

FIGS. 13A and 13B are diagrams illustrating a NO_(x) amount exhausted from the first cylinder group per unit time;

FIG. 14 is a diagram illustrating an NH₃ synthesizing efficiency of the three way catalyst;

FIG. 15 is a diagram illustrating an equivalent coefficient;

FIGS. 16A and 16B are diagrams illustrating an NH₃ amount desorbed from the NH₃ -AO catalyst per unit time;

FIG. 17 is a diagram illustrating a temperature of the exhaust gas flowing into the three way catalyst;

FIG. 18 is a diagram illustrating a temperature of the exhaust gas flowing into the NH₃ -AO catalyst;

FIG. 19 is a flow chart for executing an operation change control according to the embodiment explained with FIG. 12;

FIG. 20 is a flow chart for calculating a NO_(x) amount occluded in the NO_(x) -OR catalyst;

FIG. 21 is a time chart for explaining the exhaust gas purifying method according to further another embodiment;

FIG. 22 is a diagram illustrating a rich period value;

FIG. 23 is a flow chart for executing an operation change control in the second cylinder group according to the embodiment explained with FIG. 21;

FIG. 24 illustrates a variation of a NO_(x) amount exhaust from the engine with an engine air-fuel ratio;

FIG. 25 is a flow chart for calculating a lean air-fuel ratio;

FIG. 26 is a flow chart for controlling the operating cylinder number;

FIG. 27 is a general view of an engine, illustrating an exhaust gas purifying device according to further another embodiment;

FIG. 28 is a flow chart for control in a warming-up operation in the embodiment shown in FIG. 27;

FIG. 29 is a general view of an engine, illustrating an exhaust gas purifying device according to further another embodiment;

FIG. 30 is a flow chart for executing an operation change control in the first cylinder subgroup according to the embodiment explained with FIG. 29;

FIG. 31 is a diagram illustrating a NO_(x) amount exhausted from the first cylinder subgroup per unit time;

FIG. 32 is a diagram illustrating a NO_(x) amount passing through the NO_(x) -OR catalyst connected to the first cylinder subgroup per unit time;

FIG. 33 is a diagram illustrating a rich period value for the first cylinder subgroup;

FIG. 34 is a flow chart for executing an operation change control in the second cylinder subgroup according to the embodiment explained with FIG. 29;

FIG. 35 is a diagram illustrating a NO_(x) amount exhausted from the second cylinder subgroup per unit time;

FIG. 36 is a diagram illustrating a NO_(x) amount passing through the NO_(x) -OR catalyst connected to the second cylinder subgroup per unit time;

FIG. 37 is a diagram illustrating a rich period value for the second cylinder subgroup;

FIG. 38 illustrates a characteristic of the exhaust gas purifying catalyst according to another embodiment;

FIG. 39 illustrates another embodiment of the exhaust gas purifying catalyst;

FIG. 40 illustrates another embodiment of the exhaust gas purifying catalyst;

FIG. 41 is a flow chart for controlling an exhaust gas control valve in the embodiments shown in FIG. 40;

FIG. 42 is a general view of an engine, illustrating an exhaust gas purifying device according to still another embodiment;

FIGS. 43A, 43B, 44A, and 44B schematically illustrate the exhaust gas purifying method in the engine shown in FIG. 42;

FIG. 45 is a flow chart for executing a switching control of connecting conditions according to the embodiment shown in FIG. 42;

FIG. 46 is a general view of an engine, illustrating an exhaust gas purifying device according to still another embodiment;

FIG. 47 is a flow chart for executing an operation change control in the second cylinder group according to the embodiment explained with FIG. 46;

FIG. 48 is a general view of an engine, illustrating an exhaust gas purifying device according to still another embodiment;

FIGS. 49 and 50 schematically illustrate the exhaust gas purifying method in the engine shown in FIG. 48;

FIG. 51 is a flow chart for executing a switching control of exhaust gas air-fuel ratio conditions, according to the embodiment explained with FIG. 48; and

FIG. 52 is a general view of an engine, illustrating an alternative embodiment of the embodiment shown in FIG. 48.

BEST MODE FOR CARRYING OUT THE INVENTION

In general, nitrogen oxides (NO_(x)) include nitrogen monoxide NO, nitrogen dioxide NO₂, dinitrogen tetroxide N₂ O₄, dinitrogen monoxide N₂ O, etc. The following explanation will be made referring NO_(x) mainly as nitrogen monoxide NO and/or nitrogen dioxide NO₂, but a method and a device for purifying an exhaust gas of an engine according to the present invention can purify the other nitrogen oxides.

FIG. 1 shows the case where the present invention is applied to an internal engine of the spark ignition type. However, the present invention may be applied to a diesel engine. Also, the engine shown in FIG. 1 is used for an automobile, for example.

Referring to FIG. 1, an engine body 1 has four cylinders, i.e., a first cylinder #1, a second cylinder #2, a third cylinder #3, a fourth cylinder #4. Each cylinder #1 to #4 is connected to a common surge tank 3, via a corresponding branch 2, and the surge tank 3 is connected to a air-cleaner (not shown) via an intake duct 4. In each branch 2, a fuel injector 5 is arranged to feed fuel to the corresponding cylinder. Further, a throttle valve 6 is arranged in the intake duct 4, an opening of which becomes larger as the depression of the acceleration pedal (not shown) becomes larger. Note that the fuel injectors 5 are controlled in accordance with the output signals from an electronic control unit 20.

On the other hand, the first cylinder #1 is connected to a catalytic converter 9 housing an NH₃ synthesizing catalyst 8 therein, via an exhaust duct 7. The second, the third, and the fourth cylinders are connected to a catalytic converter 12 housing an occlusive material therein, via a common exhaust manifold 10. In the engine shown in FIG. 1, the first cylinder constructs a first cylinder group 1a, and the second, the third, and the fourth cylinders construct a second cylinder group 1b. Thus, an exhaust gas of the first cylinder group 1a is introduced to the NH₃ synthesizing catalyst 8, and that of the second cylinder group 1b is introduced to the occlusive material 11. The two catalytic converters 9 and 12 are then connected, via a common interconnecting duct 13, to a catalytic converter 15 housing an exhaust gas purifying catalyst 14 therein, and the catalytic converter 15 is connected to a catalytic converter 17 housing an NH₃ purifying catalyst 16 therein. As shown in FIG. 1, a secondary air supplying device 18 is arranged in the exhaust passage between the interconnecting passage 13 and the catalytic converter 14, for supplying a secondary air to the exhaust gas, and is controlled in accordance with the output signals from the electronic control unit 20.

The electronic control unit (ECU) 20 comprises a digital computer and is provided with a ROM (read only memory) 22, a RAM (random access memory) 23, a CPU (micro processor) 24, an input port 25, and an output port 26, which are interconnected by a bidirectional bus 21. Mounted in the surge tank 3 is a pressure sensor 27 generating an output voltage proportional to a pressure in the surge tank 3. The output voltage of the sensor 27 is input via an AD converter 28 to the input port 25. The intake air amount Q is calculated in the CPU 24 on the basis of the output signals from the AD converter 28. Further, air-fuel ratio sensors 29, 30, 31, and 32 are mounted in the exhaust duct 7, the collecting portion of the exhaust manifold 10, the interconnecting duct 13 where the exhaust gas from the second group 1b does not flow, that is, the interconnecting duct 13 just downstream of the catalytic converter 9, and the interconnecting duct 13 where the exhaust gas from the first group 1a does not flow, that is, the interconnecting duct 13 just downstream of the catalytic converter 12, respectively, each generating an output voltage proportional to an exhaust gas air-fuel ratio of the exhaust gas flowing through the corresponding portion of the exhaust passage. The output voltages of the sensors 29, 30, 31, and 32 are input via corresponding AD converters 33, 34, 35, and 36 to the input port 25. Further, connected to the input port 25 is a crank angle sensor 37 generating an output pulse whenever the crank shaft of the engine 1 turns by, for example, 30 degrees. The CPU 24 calculates the engine speed N in accordance with the pulse. On the other hand, the output port 26 is connected to the fuel injectors 5 and the secondary supplying device 18, via corresponding drive circuits 38.

In the embodiment shown in FIG. 1, the NH₃ synthesizing catalyst 8 is comprised of a three-way catalyst 8a, which is simply expressed as a TW catalyst here. The TW catalyst 8a is comprised of precious metals such as palladium Pd, platinum Pt, iridium Ir, and rhodium Rh, carried on a layer of, for example, alumina, formed on a surface of a substrate.

FIG. 2 illustrates the purifying efficiency of the exhaust gas of the TW catalyst 8a. As shown in FIG. 2, the TW catalyst 8a passes the inflowing NO_(x) therethrough when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean with respect to the stoichiometric air-fuel ratio (A/F)S, which is about 14.6 and the air-excess ratio λ=1.0, and synthesizes NH₃ from a part of the inflowing NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich. The NH₃ synthesizing function of the TW catalyst 8a is unclear, but it can be considered that some of NO_(x) in the exhaust gas of which the exhaust gas air-fuel ratio is rich is converted to NH₃ according to the following reactions (1) and (2), that is:

    5H.sub.2 +2NO→2NH.sub.3 +2H.sub.2 O                 (1)

    7H.sub.2 +2NO.sub.2 →2NH.sub.3 +4H.sub.2 O          (2)

On the contrary, it is considered that the other NO_(x) is reduced to the nitrogen N₂ according to the following reactions (3) to (6), that is:

    2CO+2NO→N.sub.2 +2CO.sub.2                          (3)

    2H.sub.2 +2NO→N.sub.2 +2H.sub.2 O                   (4)

    4CO+2NO.sub.2 →N.sub.2 +4CO.sub.2                   (5)

    4H.sub.2 +2NO.sub.2 →N.sub.2 +4H.sub.2 O            (6)

Accordingly, NO_(x) flowing in the TW catalyst 8a is converted to either NH₃ or N₂ when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and thus NO_(x) is prevented from flowing out from the Tw catalyst 8a.

As shown in FIG. 2, the efficiency ETA of the NH₃ synthesizing of the TW catalyst 8a becomes larger as the exhaust gas air-fuel ratio of the inflowing exhaust gas becomes smaller or richer than the stoichiometric air-fuel ratio (A/F)S, and is kept constant when the exhaust gas air-fuel ratio of the inflowing exhaust gas become even smaller. In the example shown in FIG. 2, the NH₃ synthesizing efficiency ETA is kept constant when the exhaust gas air-fuel ratio of the inflowing exhaust gas equals or is smaller than about 13.8, where the air-excess ratio λ is about 0.95.

On the other hand, the NO_(x) amount exhausted from each cylinder per unit time depends on the engine air-fuel ratio, as shown in FIG. 24 and explained hereinafter. In particular, the exhausted NO_(x) amount becomes smaller as the engine air-fuel ratio becomes smaller when the engine air-fuel ratio is rich. Therefore, considering the synthesizing efficiency ETA, the NH₃ amount synthesized in the TW catalyst 8a per unit time reaches the maximum amount thereof when the exhaust gas air-fuel ratio of the inflowing exhaust gas is about 13.8, if the exhaust gas air-fuel ratio of the inflowing exhaust gas conforms to the engine air-fuel ratio.

Note that, in the engine shown in FIG. 1, it is desired to synthesize as much NH₃ as possible when the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a is rich, as can be understood from the following description. Accordingly, a TW catalyst carrying palladium Pd or cerium Ce is used as the TW catalyst 8a. In particular, a TW catalyst carrying palladium Pd can also enhance the HC purifying efficiency, when the exhaust air-fuel ratio of the inflowing exhaust gas is rich. Further, note that a TW catalyst carrying rhodium Rh suppresses NH₃ synthesizing therein, and a TW catalyst without rhodium Rh is preferably used as the TW catalyst 8a.

On the other hand, the occlusive material 11 is for occluding NO_(x) in the inflowing exhaust gas to thereby prevent a large amount of NO_(x) from flowing into the exhaust gas purifying catalyst 14. The occlusive material 11 does not necessarily have a catalytic function, but, in this embodiment, a NO_(x) occluding and reducing catalyst 11a, which is simply expressed as a NO_(x) -OR catalyst, is used as the occlusive material 11. The NO_(x) -OR catalyst 11a has both an occluding and releasing function of the NO_(x) and a reducing function of NO_(x), and is comprised of at least one substance selected from alkali metals such as potassium K, sodium Na, lithium Li, and cesium Cs, alkali earth metals such as barium Ba and calcium Ca, rare earth metals such as lanthanum La and yttrium Y, and transition metals such as iron Fe and copper Cu, and of precious metals such as palladium Pd, platinum Pt, iridium Ir, and rhodium Rh, which are carried on alumina as a carrier. The NO_(x) -OR catalyst 11a performs the NO_(x) occluding and releasing function in which it occludes NO_(x) therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releases the occluded NO_(x) therefrom when the oxygen concentration in the inflowing exhaust gas becomes lower.

When the NO_(x) -OR catalyst 11a is disposed in the exhaust passage of the engine, the NO_(x) -OR catalyst 11a actually performs the NO_(x) occluding and releasing function, but the function is unclear. However, it can be considered that the function is performed according to the mechanism as explained below. This mechanism will be explained by using as an example a case where platinum Pt and barium Ba are carried on the carrier, but a similar mechanism is obtained even if another precious metal, alkali metal, alkali earth metal, or rare earth metal is used.

Namely, when the exhaust gas air-fuel ratio of the inflowing exhaust gas becomes lean, that is, when the oxygen concentration in the inflowing exhaust gas increases, the oxygen O₂ is deposited on the surface of platinum Pt in the form of O₂ ⁻ or O²⁻. On the other hand, NO in the inflowing exhaust gas reacts with the O₂ ⁻ or O²⁻ on the surface of the platinum Pt and becomes NO₂ (2NO+O₂ →2NO₂). Subsequently, a part of the produced NO₂ is oxidized on the platinum Pt and is occluded into the NO_(x) -OR catalyst 11a. While bonding with barium oxide BaO, it is diffused in the NO_(x) -OR catalyst 11a in the form of nitric acid ions NO₃ ⁻. In this way, NO_(x) is occluded in the NO_(x) -OR catalyst 11a.

Contrarily, when the oxygen concentration in the inflowing exhaust gas becomes lower and the production of NO₂ is lower, the reaction proceeds in an inverse direction (NO₃ ⁻ →NO₂), and thus nitric acid ions NO₃ ⁻ in the NO_(x) -OR catalyst 11a are released in the form of NO₂ from the NO_(x) -OR catalyst 11a. Namely, when the oxygen concentration in the inflowing exhaust gas is lowered, that is, when the exhaust gas air-fuel ratio of the inflowing exhaust gas is changed lean to rich, NO_(x) is released from the NO_(x) -OR catalyst 11a. At this time, if the reducing agent such as NH₃, HC, and CO, exists in the NO_(x) -OR catalyst 11a, NO_(x) is reduced and purified by the NH₃, the HC, and the CO.

As mentioned above, the occlusive material 11 is to prevent a large amount of NO_(x) from flowing into the exhaust gas purifying catalyst 14, and is not necessarily able to occlude all of the inflowing NO_(x) therein. Thus, the NO_(x) -OR catalyst 11a has a relatively small volume.

On the other hand, the exhaust gas purifying catalyst 14 is for purifying the inflowing NO_(x) and NH₃ simultaneously, and does not necessarily have an NH₃ adsorbing function. However, in this embodiment, the exhaust gas purifying catalyst 14 consists of an NH₃ adsorbing and oxidizing catalyst 14a, which is simply expressed as a NH₃ -AO catalyst, and has both an adsorbing and desorbing function of NH₃ and a catalytic function. The NH₃ -AO catalyst 14a is comprised of a so-called zeolite denitration catalyst, such as zeolite carrying copper Cu thereon (the Cu zeolite catalyst), zeolite carrying copper Cu and platinum Pt thereon (the PT--Cu zeolite catalyst), and zeolite carrying iron Fe thereon, which is carried on a surface of a substrate. Alternatively, the NH₃ -AO catalyst 14a may be comprised of solid acid such as zeolite, silica, silica-alumina, and titania, carrying the transition metals such as iron Fe and copper Cu or precious metals such as palladium Pd, platinum Pt, iridium Ir, and rhodium Rh, or of a combination of at least two of the above. Further alternatively, the exhaust gas purifying catalyst 14 may be comprised of a catalyst carrying at least precious metals (precious metal catalyst), or of a combination of the precious metal catalyst and the NH₃ -AO catalyst.

It is considered that the NH₃ -AO catalyst 14a adsorbs NH₃ in the inflowing exhaust gas, and desorbs the adsorbed NH₃ when the NH₃ concentration in the inflowing exhaust gas becomes lower, or when the inflowing exhaust gas includes NO_(x). At this time, it is considered that, if the NH₃ -AO catalyst 14a is under the oxidizing atmosphere, that is, if the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, the NH₃ -AO catalyst 14a oxidizes all of NH₃ desorbed therefrom. Or, it is considered that, if the inflowing exhaust gas includes both NH₃ and NO_(x), the NH₃ is oxidized by the NO_(x) on the NH₃ -AO catalyst 14a. In these cases, the NH₃ oxidizing function is partly unclear, but it can be considered that the NH₃ oxidation occurs according to the following reactions (7) to (10), that is:

    4NH.sub.3 +7O.sub.2 →4NO.sub.2 +6H.sub.2 O          (7)

    4NH.sub.3 +5O.sub.2→4 NO+6H.sub.2 O                 (8)

    8NH.sub.3 +6NO.sub.2 →12H.sub.2 O+7N.sub.2          (9)

    4NH.sub.3 +4NO+O.sub.2 →6H.sub.2 O+4N.sub.2         (10)

The reactions (9) and (10), which are denitration, reduce both NO_(x) produced in the oxidation reactions (7) and (8), and NO_(x) in the exhaust gas flowing in the NH₃ -AO catalyst 14a. Note that, alternatively, there may be provided the exhaust gas purifying catalyst 14 and the adsorbent separated from each other, and the adsorbent may be arranged downstream of the catalyst 14.

The NH₃ purifying catalyst 16 is comprised of transition metals such as iron Fe and copper Cu, or precious metals such as palladium Pd, platinum Pt, iridium Ir, and rhodium Rh, carried on a layer of, for example, alumina, formed on a surface of a substrate. The NH₃ purifying catalyst 12 purifies or resolves NH₃ in the inflowing exhaust gas, if the catalyst 12 is under the oxidizing atmosphere, that is, if the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean. In this case, it is considered that the oxidation and denitration reactions (7) to (10) mentioned above occur in the catalyst 12 and thereby NH₃ is purified or resolved. In this embodiment and embodiments described below, basically, the NH₃ amount exhausted from the NH₃ -AO catalyst 14a is kept at zero, but the NH₃ purifying catalyst 16 ensures preventing NH₃ from being emitted to the ambient air, even if NH₃ is included in the inflowing exhaust gas.

In the engine shown in FIG. 1, the fuel injection time TAU is calculated using the following equation:

    TAU=TB·((A/F)S/(A/F)T)·FAF

TB represents a basic fuel injection time suitable for making the engine air-fuel ratio of each cylinder equal to the stoichiometric air-fuel ratio (A/F)S, and is calculated using the following equation:

    TB=(Q/N)·K

where Q represents the intake air amount, N represents the engine speed, and K represent a constant. Accordingly, the basic fuel injection time TB is a product of an intake air amount per unit engine speed, and the constant.

(A/F)T represents a target value for the control of the engine air-fuel ratio. When the target value (A/F)T is made larger to make the engine air-fuel ratio lean with respect to the stoichiometric air-fuel ratio, the fuel injection time TAU is made shorter and thereby the fuel amount to be injected is decreased. When the target value (A/F)T is made smaller to make the engine air-fuel ratio rich with respect to the stoichiometric air-fuel ratio, the fuel injection time TAU is made longer and thereby the fuel amount to be injected is increased. Note that, in this embodiment, the target values for the cylinders of the second cylinder group 1b are made identical to each other.

FAF represents a feedback correction coefficient for making the actual engine air-fuel ratio equal to the target value (A/F)T. When calculating the fuel injection time TAU for the cylinder of the first cylinder group 1a, that is, for the first cylinder #1, FAFA is memorized as the feedback correction coefficient FAF, and when calculating the fuel injection time TAU for each cylinder of the second cylinder group 1b, that is, for the second, the third, and the fourth cylinders, FAFB is memorized as the feedback correction coefficient FAF. The feedback correction coefficients FAFA and FAFB are determined, mainly, on the basis of the output signals from the air-fuel ratio sensors 29 and 30, respectively. The exhaust gas air-fuel ratio of the exhaust gas flowing through the exhaust duct 7 and detected by the sensor 29 conforms to the engine air-fuel ratio of the first group 1a. When the exhaust gas air-fuel ratio detected by the sensor 29 is lean with respect to the target value (A/F)T for the first group 1a, the feedback correction coefficient FAFA is made larger and thereby the fuel amount to be injected is increased. When the exhaust gas air-fuel ratio detected by the sensor 29 is rich with respect to the target value (A/F)T for the first group 1a, FAFA is made smaller and thereby the fuel amount to be injected is decreased. In this way, the actual engine air-fuel ratio of the first group 1a is made equal to the target value (A/F)T for the first group 1a.

Also, the exhaust gas air-fuel ratio of the exhaust gas flowing through the exhaust manifold 10 and detected by the sensor 30 conforms to the engine air-fuel ratio of the second group 1b. When the exhaust gas air-fuel ratio detected by the sensor 30 is lean with respect to the target value (A/F)T for the second group 1b, the feedback correction coefficient FAFB is made larger and, thereby, the fuel amount to be injected is increased. When the exhaust gas air-fuel ratio detected by the sensor 30 is rich with respect to the target value (A/F)T for the second group 1b, FAFB is made smaller and, thereby, the fuel amount to be injected is decreased. In this way, the actual engine air-fuel ratio of the second group 1b is made equal to the target value (A/F)T for the second group 1b. Note that the feedback correction coefficients FAFA and FAFB fluctuate around 1.0, respectively.

The air-fuel ratio sensors 31 and 32 are for making the actual engine air-fuel ratio equal to the target value more precisely. Namely, the sensors 31 and 32 are for compensating for the deviation of the engine air-fuel ratio of the first and second groups 1a and 1b from the corresponding target value (A/F)T due to the deterioration of the sensors 29 and 30. As each sensor 29, 30, 31, 32, a sensor suitably selected from an air-fuel ratio sensor generating an output voltage which corresponds to the exhaust gas air-fuel ratio over the broader range of the exhaust gas air-fuel ratio, and a Z-output type oxygen concentration sensor, of which an output voltage varies drastically when the detecting exhaust gas air-fuel ratio increases or decreases across the stoichiometric air-fuel ratio, may be used.

In the engine shown in FIG. 1, there is no device for supplying secondary fuel or secondary air in the exhaust passage, other than the secondary air supplying device 18. Thus, the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a conforms to the engine air-fuel ratio of the first group 1a, and the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a conforms to the engine air-fuel ratio of the second group 1b. Contrarily, in the exhaust passage downstream of the secondary air supplying device 18, the exhaust gas air-fuel ratio conforms to a ratio of the total amount of air fed into all of the cylinders to the total amount of fuel fed into all of the cylinders when the supply of the secondary air is stopped, and is made lean with respect to that ratio when the secondary air is supplied.

Next, the exhaust gas purifying method in this embodiment will be explained with reference to FIGS. 3 and 4.

In this embodiment, the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a is basically made rich, and an exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is basically made lean. When the exhaust gas air-fuel ratio of the inflowing exhaust gas is made rich, the TW catalyst 8a converts a part of the inflowing NO_(x). The NH₃ synthesized in the TW catalyst 8a then flows into the NH₃ -AO catalyst 14a, via the interconnecting duct 13. On the other hand, when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is made lean, most NO_(x) in the inflowing exhaust gas is occluded in the NO_(x) -OR catalyst 11a, and the remaining NO_(x) passes through the NO_(x) -OR catalyst 11a without being occluded. The NO_(x) then flows into the NH₃ -AO catalyst 14a, via the interconnecting duct 13.

Into the NH₃ -AO catalyst 14a is mixed the exhaust gas exhausted from the TW catalyst 8a and that from the NO_(x) -OR catalyst 11a. The exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a is kept lean in this embodiment, and thus the NO_(x) and the NH₃ are purified according to the above-mentioned reactions (7) to (10), on the NH₃ -AO catalyst 14a. Therefore, NO_(x) and NH₃ are prevented from being emitted to the ambient air. Note that, when the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a is rich, unburned hydrocarbon HC, carbon monoxide CO, or hydrogen H₂ may pass through the TW catalyst 8a and may flow into the NH₃ -AO catalyst 14a. It is considered that the HC, CO, etc. act as the reducing agent, as well as NH₃, and reduce a part of NO_(x) on the NH₃ -AO catalyst 14a. However, the reducing ability of NH₃ is higher than those of HC, CO, etc., and thus NO_(x) can be reliably purified by using NH₃ as the reducing agent.

As mentioned above, the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a conforms to the engine air-fuel ratio of the first cylinder group 1a. Thus, to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a rich, the first group 1a performs a rich operation in which the engine air-fuel ratio of each cylinder is rich with respect to the stoichiometric air-fuel ratio (A/F)S. In other words, if the target value (A/F)T of the engine air-fuel ratio of each cylinder is referred as a target air-fuel ratio, the target air-fuel ratio (A/F)T of the first cylinder #1 is made equal to a rich air-fuel ratio (A/F)R which is rich with respect to the stoichiometric air-fuel ratio (A/F)S, to thereby make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a rich.

Also, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a conforms to the engine air-fuel ratio of the second cylinder group 1b. Thus, to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a lean, the second group 1b performs a lean operation in which the engine air-fuel ratio of each cylinder is lean with respect to the stoichiometric air-fuel ratio (A/F)S. In other words, the target air-fuel ratio (A/F)T of each of the second, the third, and the fourth cylinders is made equal to a lean air-fuel ratio (A/F)L which is lean with respect to (A/F)S, to thereby make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a lean.

Note that, to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a rich, a secondary fuel supplying device for supplying secondary fuel into the exhaust duct 7 may be provided, and may supply secondary fuel while the first group 1a performs the lean operation. Further, note that, to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a lean, a secondary air supplying device for supplying secondary air into the exhaust manifold 10 may be provided, and may supply secondary air while the second group 1b performs the rich operation.

The lean air-fuel ratio (A/F)L and the rich air-fuel ratio (A/F)R may be set to vary in accordance with the engine operating condition, respectively. However, in this embodiment, the lean air-fuel ratio (A/F)L is set constant at about 18.5, and the rich air-fuel ratio (A/F)R is set constant at about 13.8, regardless the engine operating condition. Therefore, the target air-fuel ratio (A/F)T of the first cylinder is kept constant at about 18.5, and that of each of the second, the third, and the fourth cylinders is kept constant at about 13.8. By setting the lean and the rich air-fuel ratios (A/F)L and (A/F)R in the above-mentioned manner, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a is kept lean, without supplying the secondary air supplying device 18. Further, by setting the rich air-fuel ratio (A/F)R to about 13.8, a large amount of NH₃ is synthesized in the TW catalyst 8a.

If a cylinder operates the lean operation, the fuel consumption rate is lowered. Thus, when the second group 1b basically performs the lean operation, as in the embodiment, the fuel consumption rate of the engine 1 can be lowered, while purifying the exhaust gas sufficiently. In particular, in the engine shown in FIG. 1, the number of the second group 1b is larger than half of the total cylinder number of the engine 1, and thus the fuel consumption rate is further lowered while purifying the exhaust gas sufficiently.

As the cylinder number of the second group 1b becomes larger, that is, as the number of the cylinder performing the lean operation becomes larger, the fuel consumption rate becomes lower. However, as the cylinder number of the second group 1b becomes larger, the NO_(x) amount exhausted from the second group 1b becomes larger. If such a large amount of NO_(x) is introduced to the NH₃ -AO catalyst 14a directly, that is, without contacting the NO_(x) -OR catalyst 11a, the NO_(x) may be emitted from the NH₃ -AO catalyst 14a without being reduced, because there may a case where NH₃ required to purify such a large amount of NO_(x) is not supplied to the NH₃ -AO catalyst 14a. Namely, the NO_(x) amount may be excessive to the NH₃ amount, at the NH₃ -AO catalyst 14a. In particular, as the cylinder number of the second group 1b becomes larger, that of the first group 1a becomes smaller and the NO_(x) amount exhausted from the first cylinder 1a becomes smaller. As a result, as the cylinder number of the second group 1b becomes larger, the NH₃ amount flowing into the NH₃ -AO catalyst 14a becomes smaller. Thus, in this case, there is a large possibility that the NO_(x) amount is excessive to the NH₃ amount in the NH₃ -AO catalyst 14a.

Thus, in this embodiment, the exhaust gas from the second cylinder group 1b is introduced to the NO_(x) -OR catalyst 11a to thereby occlude most of the NO_(x) in the NO_(x) -OR catalyst 11a and suppress the NO_(x) amount flowing into the NH₃ -AO catalyst 14a, to thereby prevent the NO_(x) amount flowing into the NH₃ -AO catalyst 14a from exceeding a NO_(x) amount which can be purified by the NH₃ flowing into the NH₃ -AO catalyst 14a. In other words, the NO_(x) amount is made equal to or smaller than a NO_(x) amount which can be purified by the NH₃ flowing into the NH₃ -AO catalyst 14a. As a result, substantially all of NO_(x) flowing into the NH₃ -AO catalyst 14a is purified sufficiently.

If controlling one or both of the NH₃ amount and the NO_(x) amount flowing into the NH₃ -AO catalyst 14a to react NH₃ and NO_(x) without any excess and any lack, no NH₃ and NO_(x) may flow out from the NH₃ -AO catalyst 14a. However it is difficult to controlling one or both of the NH₃ amount and the NO_(x) amount flowing into the NH₃ -AO catalyst 14a precisely. Contrarily, in this embodiment, the NO_(x) amount flowing into the NH₃ -AO catalyst 14a is merely suppressed, and thus the controllability and the structure of the device are simplified.

On the other hand, when the NO_(x) -OR catalyst 11a deteriorates and the occluding ability is lowered, or when the amount or the concentration of NO_(x) flowing into the NO_(x) -OR catalyst 11a widely increases, an undesirable leakage of NO_(x) from the NO_(x) -OR catalyst 11a may occur, even when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean. However, the leaked NO_(x) then flows into the NH₃ -AO catalyst 14a, and is reduced by NH₃. Accordingly, even if the undesirable leakage of NO_(x) from the NO_(x) -OR catalyst 11a occurs, the NO_(x) is prevented from being emitted to the ambient air.

When the NH₃ amount flowing into the NH₃ -AO catalyst 14a is excessive to the NO_(x) amount flowing into the NH₃ -AO catalyst 14a, the excess NH₃ is adsorbed in the NH₃ -AO catalyst 14a. Thus, NH₃ is prevented from being emitted to the ambient air. Further, in this embodiment, the NH₃ purifying catalyst 16 is arranged downstream of the NH₃ -AO catalyst 14a. Thus, even if NH₃ flows out from the NH₃ -AO catalyst 14a without being adsorbed, the NH₃ is purified on the NH₃ purifying catalyst 16. In this way, NH₃ is reliably prevented from being emitted to the ambient air. The exhaust gas purifying method described above is schematically illustrated in FIG. 3.

If the second group 1b continuously performs the lean operation, the fuel consumption rate is further lowered. However, if the second group 1b continuously performs the lean operation, the occluding capacity of the NO_(x) -OR catalyst 11a becomes lower. If the NO_(x) -OR catalyst 11a is saturated with NO_(x), the relatively large amount of NO_(x) exhausted from the second group 1b flows into the NH₃ -AO catalyst 14a directly. On the other hand, when the exhaust gas air-fuel ratio of the inflowing exhaust gas is made rich, the NO_(x) -OR catalyst 11a releases the occluded NO_(x) therefrom, as mentioned above. Thus, in this embodiment, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is changed to rich temporarily to release the occluded NO_(x) from the NO_(x) -OR catalyst 11a, to thereby prevent the catalyst 11a from being saturated with NO_(x). Accordingly, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is made lean and rich alternately and repeatedly.

To make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a rich temporarily, a secondary fuel supplying device for supplying secondary fuel into the NO_(x) -OR catalyst 11a may be provided, and may supply secondary fuel temporarily while the second group 1b performs the lean operation. However, as mentioned above, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a conforms to the engine air-fuel ratio of the second cylinder group 1b. Therefore, in this embodiment, the second group 1b performs the rich operation temporarily, to thereby make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a rich temporarily. Namely, the target air-fuel ratio (A/F)T of for the second group 1b is temporarily set to a rich air-fuel ratio (A/F)RR. The rich air-fuel ratio (A/F)RR may be set to any air-fuel ratio, but in this embodiment, is set to about 13.8 regardless the engine operating condition. Accordingly, the second group 1b performs the lean and the rich operations alternately and repeatedly.

Almost of the NO_(x) released from the NO_(x) -OR catalyst 11a and the NO_(x) flowing from the second group 1b to the NO_(x) -OR catalyst 11a when the second group 1b performs the rich operation is reduced on the NO_(x) -OR catalyst 11a by HC and CO flowing into the catalyst 11a and the NH₃ synthesized in the catalyst 11a. Namely, the NO_(x) -OR catalyst 11a is a catalyst produced by adding barium, for example, to a three way catalyst, as can be seen the catalytic components mentioned above. Thus, when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, the NO_(x) -OR catalyst 11a converts NO_(x) on the catalyst 11a to NH₃. The NH₃ reduces NO_(x) on the NO_(x) -OR catalyst 11a immediately, or it flows into the NH₃ -AO catalyst 14a.

The small amount of NO_(x) flowing out from the NO_(x) -OR catalyst 11a when the second group 1b performs the rich operation then flows into the NH₃ -AO catalyst 14a.

When the second group 1b performs the rich operation and the exhaust gas air-fuel ratio of the inflowing exhaust gas is made rich, the occluded NO_(x) is released from the NO_(x) -OR catalyst 11a almost at once. However, just after the exhaust gas of which the exhaust gas air-fuel ratio is rich flows into the NO_(x) -OR catalyst 11a, the amount of the reducing agent is still small on the NO_(x) -OR catalyst 11a, and thus some of the NO_(x) on the NO_(x) -OR catalyst 11a escapes from the catalyst 11a without being reduced. Further, the inventors of the present invention has found that, when CO, CO₂, H₂ O, etc. are not present, NO_(x) in the form of NO₂ easily reacts with NH₃, but NO_(x) in the form of NO hardly reacts with NH₃, as long as O₂ is not present. As mentioned above, NO_(x) is released from the NO_(x) -OR catalyst 11a in the form of NO₂. However, if the NO₂ is converted to NO on the NO_(x) -OR catalyst 11a, the NO is hardly converted to NO₂, because the oxygen concentration on the catalyst 11a is very low at this time. As mentioned above, the NO hardly reacts with NH₃. Accordingly, the NO_(x) in the form of NO also escapes from the NO_(x) -OR catalyst 11a.

Note that the NO_(x) amount escaping from the NO_(x) -OR catalyst 11a at the beginning of the rich operation of the second group 1b becomes larger, as the occluded NO_(x) amount becomes larger, as the temperature of the catalyst 11a becomes higher, and as the rich air-fuel ratio (A/F)RR becomes larger, that is, becomes closer to the stoichiometric air-fuel ratio (A/F)S.

The NO_(x), escaping from the NO_(x) -OR catalyst 11a at the beginning of the rich operation of the second group also flows into the NH₃ -AO catalyst 14a.

If the first group 1a performs the rich operation when the second group 1b performs the rich operation, the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a is made rich, and thus NO_(x) may be reduced in the NH₃ -AO catalyst 14a sufficiently, even if NH₃ is desorbed therefrom or is fed from the TW catalyst 8a, because the catalyst 14a is not in an oxidizing atmosphere. Therefore, the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a is made lean when the second group 1b has to perform the rich operation to make the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a lean, to thereby keep the NH₃ -AO catalyst 14a under the oxidizing atmosphere. Accordingly, the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a is made rich and lean alternately and repeatedly.

To make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a lean temporarily, a secondary air supplying device for supplying secondary air into the TW catalyst 8a may be provided, and may supply secondary air temporarily while the first group 1a performs the rich operation. However, as mentioned above, the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a conforms to the engine air-fuel ratio of the first cylinder group 1a. Therefore, in this embodiment, the first group 1a performs the lean operation temporarily, to thereby make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a lean temporarily. Namely, the target air-fuel ratio (A/F)T of for the first group 1a is temporarily set to a lean air-fuel ratio (A/F)LL. The lean air-fuel ratio (A/F)LL may be set to any air-fuel ratio, but in this embodiment, is set to about 18.5 regardless the engine operating condition. Accordingly, the first group 1a performs the rich and the lean operations alternately and repeatedly.

When the first group 1a performs the lean operation, NO_(x) in the inflowing exhaust gas passes through the TW catalyst 8a. That is, the inflowing NO_(x) flows out without being converted to either NH₃ or N₂. The NO_(x) then flows into the NH₃ -AO catalyst 14a. At this time, the NH₃ concentration in the inflowing exhaust gas is low or the exhaust gas includes NO_(x), and thus NH₃ is desorbed from the NH₃ -AO catalyst 14a. At this time, the NH₃ -AO catalyst 14a is under the oxidizing atmosphere, and thus the desorbed NH₃ reduces and purifies NO_(x) in the inflowing exhaust gas. Accordingly, NO_(x) and NH₃ in the exhaust gas flowing into the NH₃ -AO catalyst 14a are purified, when the first group 1a performs the lean operation and the second group 1b performs the rich operation. Note that NO_(x) in the form of NO is also purified sufficiently in the NH₃ -AO catalyst 14a.

Even if the NH₃ amount desorbed from the NH₃ -AO catalyst 14a exceeds the amount required for reducing the inflowing NO_(x) when the first group 1a performs the lean operation and the second group 1b performs the rich operation, the excess NH₃ is purified or resolved in the following NH₃ purifying catalyst 16. Accordingly, NH₃ is prevented from being emitted to the ambient air. The exhaust gas purifying method in this case is illustrated in FIG. 4.

As mentioned above, when the rich operation of the first group 1a is stopped temporarily, the NH₃ synthesizing of the TW catalyst 8a is also temporarily stopped, and the NH₃ flowing into the NH₃ -AO catalyst 14a is temporarily stopped. As a result, the adsorbed NH₃ is desorbed from the NH₃ -AO catalyst 14a by causing the first group 1a to perform the lean operation. Accordingly, by causing the first group 1a to perform the lean operation, the NH₃ adsorbing capacity of the NH₃ -AO catalyst 14a is also ensured.

Note that the lean and the rich air-fuel ratios (A/F)LL and (A/F)RR are set to make the exhaust gas air-fuel ratio of the exhaust gas mixture flowing, via the interconnecting duct 13, into the NH₃ -AO catalyst 14a lean. However, there may be a case where the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a is made rich at a transition engine operation. Thus, to make the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a lean or stoichiometric regardless of the engine operating condition, the engine shown in FIG. 1 comprises the secondary air supplying device 18. The secondary air supplying device 18 supplies the secondary air to the NH₃ -AO catalyst 14a continuously or intermittently.

As long as the NO_(x) -OR catalyst 11a is prevented from being saturated, any method may be applied for determining a timing at which the operations of the first and the second groups 1a and 1b are changed between the rich and the lean operations. In this embodiment, this operation change control is performed in accordance with the NO_(x) amount occluded in the NO_(x) -OR catalyst 11a. Namely, the occluded NO_(x) amount S(NO_(x)) is obtained, and the operation of the first group 1a is changed from the rich to the lean and that of the second group 1b is changed from the lean to the rich, when the occluded NO_(x) amount S(NO_(x)) becomes larger than a predetermined upper threshold amount UT(NO_(x)). When the occluded NO_(x) amount S(NO_(x)) becomes smaller than a predetermined lower threshold amount LT(NO_(x)), the operation of the first group 1a is changed from the lean to the rich and that of the second group 1b is changed from the rich to the lean.

Changing the operations of the first and the second groups 1a and 1b when the occluded NO_(x) amount S(NO_(x)) becomes larger than the upper threshold amount UT(NO_(x)), or becomes lower than the lower threshold amount LT(NO_(x)), as mentioned above, can decrease the frequency of the operation change.

FIG. 5 shows a time chart illustrating the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a, and the target air-fuel ratios for the first and the second groups 1a and 1b. In FIG. 5, the time zero represents a time when the first and the second groups 1a and 1b start the rich and the lean operations, respectively. When the first group 1a performs the rich operation with the target air-fuel ratio (A/F)T being the rich air-fuel ratio (A/F)R, and the second group 1b performs the lean operation with the target air-fuel ratio (A/F)T being the lean air-fuel ratio (A/F)L, the occluded NO_(x) amount S(NO_(x)) becomes larger, and is larger than the upper threshold amount UT(NO_(x)) at the time a. When S(NO_(x))>UT(NO_(x)), the target air-fuel ratio (A/F)T for the first group 1a is set to the lean air-fuel ratio (A/F)LL, and that for the second group 1b is set to the rich air-fuel ratio (A/F)RR. As a result, the occluded NO_(x) is released and the occluded NO_(x) amount S(NO_(x)) becomes smaller. At the time b, the occluded NO_(x) amount S(NO_(x)) is smaller than the lower threshold LT(NO_(x)), and the target air-fuel ratios (A/F)T for the first and the second groups 1a and 1b are set again to the rich and the lean air-fuel ratio (A/F)R and (A/F)L, respectively.

It is difficult to directly find the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a. Therefore, in this embodiment, the occluded NO_(x) amount S(NO_(x)) is estimated on the basis of the NO_(x) amount flowing into the NO_(x) -OR catalyst 11a, that is, the NO_(x) amount exhausted from the second group 1b, and of the NO_(x) amount F(NO_(x)) passing through the NO_(x) -OR catalyst 11a. In this case, a sensor for detecting the NO_(x) amount flowing into the NO_(x) -OR catalyst 11a may be arranged in, for example, the exhaust manifold 10 between the second group 1b and the NO_(x) -OR catalyst 11a. However, the NO_(x) amount flowing into the NO_(x) -OR catalyst 11a can be found on the basis of the engine operating condition. Namely, as the engine speed N becomes higher, the NO_(x) amount exhausted from the second cylinder 1b per unit time becomes larger and thus the NO_(x) amount flowing into the NO_(x) -OR catalyst 11a per unit time becomes larger. Also, the exhaust gas amount exhausted from the second group 1b becomes larger and the combustion temperature becomes higher as the engine load Q/N (the intake air amount Q/the engine speed N) becomes higher, and thus the NO_(x) amount flowing into the TW catalyst 8a per unit becomes larger as the engine load Q/N becomes higher.

FIG. 6A illustrates the relationships, obtained by experiment, between the NO_(x) amount exhausted from the second group 1b per unit time Qb(NO_(x)), the engine load Q/N, and the engine speed N, with the constant lean air-fuel ratio (A/F)L. In FIG. 6A, the curves show the identical NO_(x) amounts. As shown in FIG. 6A, the exhausted NO_(x) amount Qb(NO_(x)) becomes larger as the engine load Q/N becomes higher, and as the engine speed N becomes higher. Note that the exhausted NO_(x) amount Qb(NO_(x)) is stored in the ROM 22 in advance in the form of a map as shown in FIG. 6B.

For detecting the NO_(x) amount F(NO_(x)) passing through the NO_(x) -OR catalyst 11a and flowing into the NH₃ -AO catalyst 14a, a sensor may be arranged in the interconnecting duct 13 between the NO_(x) -OR catalyst 11a and NH₃ -AO catalyst 14a. However, the inflowing NO_(x) amount F(NO_(x)) can be found on the basis of the NO_(x) amount flowing into the NO_(x) -OR catalyst 11a, that is, the exhausted NO_(x) amount Qb(NO_(x)), and of the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a.

FIG. 7A illustrates experimental results of the NO_(x) amount passing through the NO_(x) -OR catalyst 11a per unit time F(NO_(x)). In FIG. 7A, the curves show the identical NO_(x) amounts. As shown in FIG. 7A, the passing NO_(x) amount F(NO_(x)) becomes larger as the exhausted NO_(x) amount Qb(NO_(x)) becomes larger, and F(NO_(x)) becomes larger as the occluded NO_(x) amount S(NO_(x)) becomes larger. Note that the passing NO_(x) amount F(NO_(x)) is stored in the ROM 22 in advance in the form of a map as shown in FIG. 7B.

Namely, when the second group 1b performs the lean operation, the occluded NO_(x) amount S(NO_(x)) increases by Qb(NO_(x))-F(NO_(x)) per unit time. Thus, when the second group 1b performs the lean operation, the occluded NO_(x) amount S(NO_(x)) is calculated using the following equation:

    S(NO.sub.x)=S(NO.sub.x)+{Qb(NO.sub.x)-F(NO.sub.x)}·DELTAna

where DELTAna represents the time interval of the detection of Qb(NO_(x)). Thus, {Qb(NO_(x))-F(NO_(x))}·DELTAna represents the NO_(x) amount occluded in the NO_(x) -OR catalyst 11a from the last detection of Qb(NO_(x)) until the present detection.

FIG. 8A illustrates the NO_(x) amount D(NO_(x)) released from the NO_(x) -OR catalyst 11a per unit time, obtained by experiment. In FIG. 8A, the solid curve shows the case where the temperature TNC of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is high, and the broken curve shows the case where the exhaust gas temperature TNC is low. The exhaust gas temperature TNC represents the temperature of the NO_(x) -OR catalyst 11a. Further, in FIG. 8A, TIME represents a time at which the second group 1b starts the rich operation, that is, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is changed from lean to rich. The decomposition rate of NO_(x) in the NO_(x) -OR catalyst 11a becomes higher as the temperature of the catalyst 11a becomes higher. Thus, when the exhaust gas temperature TNC is high as shown by the solid line in FIG. 8A, a large amount of NO_(x) is released from the NO_(x) -OR catalyst 11a in a short time, while when TNC is low, as shown by the broken line in FIG. 8A, a small amount of NO_(x) is released. In other words, the released NO_(x) amount per unit time D(NO_(x)) becomes larger as the exhaust gas temperature TNC becomes higher. The released NO_(x) amount D(NO_(x)) is stored in the ROM 22 as a function of TNC and TIME, in advance in the form of a map as shown in FIG. 8B.

While the exhaust gas temperature TNC may be detected by using a temperature sensor arranged in the exhaust passage, TNC is estimated on the basis of the engine load Q/N and the engine speed N, in this embodiment. That is, TNC is obtained in advance by experiment and is stored in the ROM 22 in advance in the form of a map as shown in FIG. 9.

Namely, when the second group 1b performs the rich operation, the occluded NO_(x) amount S(NO_(x)) decreases by D(NO_(x)) per unit time. Thus, when the second group 1b performs the rich operation, the occluded NO_(x) amount S(NO_(x)) is calculated using the following equation:

    S(NO.sub.x)=S(NO.sub.x)-D(NO.sub.x)·DELTAnd

where DELTAnd represents the time interval of the detection of D(NO_(x)). Thus, D(NO_(x))·DELTAnd represents the NO_(x) amount released from the NO_(x) -OR catalyst 11a from the last detection of D(NO_(x)) until the present detection.

Note that, when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is made rich, the exhaust gas air-fuel ratio of the exhaust gas flowing out from the catalyst 11a is substantially stoichiometric when the occluded NO_(x) is released and reduced, and becomes rich when the releasing of NO_(x) has been completed. Thus, when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is to be made rich, the exhaust gas air-fuel ratio may be kept rich as long as the exhaust gas air-fuel detected by the sensor 32 is substantially stoichiometric, and may be changed to lean when the exhaust gas air-fuel detected by the sensor 32 changes to rich.

If a uniform air-fuel mixture spreads over the entire combustion chamber when the engine air-fuel ratio is very lean, such as 18.5, the spark plug (not shown) cannot ignite the air-fuel mixture, because the air-fuel mixture is very thin, and misfiring may occur. To solve this, in the engine shown in FIG. 1, an ignitable air-fuel mixture is formed in a restricted region in the combustion chamber and the reminder is filled with only the air or only the air and the EGR gas, and the air-fuel mixture is ignited by the spark plug, when the lean engine operation is to be performed. This prevents the engine from misfiring, even though the engine air-fuel ratio is very lean. Alternatively, the misfiring may be prevented by forming the swirl flow in the combustion chamber, while forming a uniform air-fuel mixture in the combustion chamber.

FIG. 10 illustrates a routine for executing the operation change control, mentioned above. The routine is executed by interruption every predetermined time.

Referring to FIG. 10, first, in step 40, it is judged whether a NO_(x) release flag is set. The NO_(x) release flag is set when the lean and the rich operations are to be performed in the first and the second groups 1a and 1b, respectively, to release NO_(x) from the NO_(x) -OR catalyst 11a, and is reset when the rich and the lean operations are to be performed in the first and the second groups 1a and 1b, respectively. If the NO_(x) release flag is reset, the routine goes to step 41, where Qb(NO_(x)) is calculated using the map shown in FIG. 6B. In the following step 41a, F(NO_(x)) is calculated using the map shown in FIG. 7B. In the following step 42, the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a is calculated using the following equation:

    S(NO)=S(NO.sub.x)+{Qb(NO.sub.x)-F(NO.sub.x)}·DELTAna

where DELTAna is a time interval from the last processing cycle until the present processing cycle. In the following step 43, it is judged whether the occluded NO_(x) amount S(NO_(x)) is larger than the upper threshold amount UT(NO_(x)). If S(NO_(x))≦UT(NO_(x)), the processing cycle is ended. Namely, if S(NO_(x))≦UT(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to be still large, and thus the first and the second groups 1a and 1b continuously perform the rich and the lean operations, respectively.

If S(NO_(x))>UT(NO_(x)) in step 43, the routine goes to step 44, where the NO_(x) release flag is set, and then the processing cycle is ended. Namely, if S(NO_(x))>UT(NO_(x)), the NO_(x) occluding capacity is judged to become small. Thus, the first group 1a stops the rich operation and starts the lean operation, and the second group 1b stops the lean operation and starts the rich operation.

Contrarily, if the NO_(x) release flag is set, the routine goes from step 40 to step 45, where the exhaust gas temperature TNC is calculated using the map shown in FIG. 9. In the following step 46, the desorbed NH₃ amount D(NH₃) is calculated using the map shown in FIG. 8B. In the following step 47, the occluded NO_(x) amount S(NO_(x)) is calculated using the following equation:

    S(NO.sub.x)=S(NO.sub.x)-D(NO.sub.x)·DELTAnd

where DELTAnd is a time interval from the last processing cycle until the present processing cycle. In the following step 48, it is judged whether the occluded NO_(x) amount S(NO_(x)) is smaller than the lower threshold amount LT(NO_(x)). If S(NO_(x))≧LT(NO_(x)), the processing cycle is ended. Namely, if S(NO_(x))≧LT(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to be still small, and thus the first and the second groups 1a and 1b continuously perform the lean and the rich operations, respectively.

If S(NO_(x))<LT(NO_(x)), the routine goes to step 48, the NO_(x) release flag is reset and the processing cycle is ended. Namely, if S(NO_(x))<LT(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to be sufficiently large. Thus, the first group 1a stops the lean operation and starts the rich operation, and the second group 1b stops the rich operation and starts the lean operation.

FIG. 11 illustrates the routine for calculating the fuel injection time TAU. The routine is executed by interruption every predetermined crank angle.

Referring to FIG. 11, first, in step 60, the basic fuel injection time TB is calculated using the following equation, on the basis of the engine load Q/N and the engine speed N:

    TB=(Q/N)·K

In the following step 61, it is judged whether the fuel injection time TAU to be calculated in this processing cycle is for the first group 1a or for the second group 1b. If TAU is for the first group 1a, that is, for the first cylinder #1, the routine goes to step 62, where the feedback correction coefficient for the first group 1a FAFA is calculated. In the following step 63, FAFA is memorized as FAF. In the following step 64, it is judged whether the NO_(x) release flag, which is set or reset in the routine shown in FIG. 10, is set. If the NO_(x) release flag is set, that is, if the lean operation is to be performed in the first group 1a, the routine goes to step 65, where the lean air-fuel ratio (A/F)LL is memorized as the target air-fuel ratio (A/F)T. In this embodiment, the lean air-fuel ratio (A/F)LL is kept constant at 18.5 regardless the engine operating condition, and thus the target air-fuel ratio (A/F)T is made 18.5 in step 65. Next, the routine goes to step 72.

Contrarily, if the NO_(x) release flag is reset, that is, if the rich operation is to be performed in the first group 1a, the routine goes to step 66, where the rich air-fuel ratio (A/F)R is memorized as the target air-fuel ratio (A/F)T. In this embodiment, the rich air-fuel ratio (A/F)R is kept constant at 13.8 regardless the engine operating condition, and thus the target air-fuel ratio (A/F)T is made 13.8 in step 66. Next, the routine goes to step 72.

If TAU is for the second group 1b in step 61, that is, for any one of the second, the third, and the fourth cylinders, the routine goes to step 67, where the feedback correction coefficient for the second group 1b FAFB is calculated. In the following step 68, FAFB is memorized as FAF. In the following step 69, it is judged whether the NO_(x) release flag is set. If the NO_(x) release flag is set, that is, if the rich operation is to be performed in the second group 1b, the routine goes to step 70, where the rich air-fuel ratio (A/F)RR is memorized as the target air-fuel ratio (A/F)T. In this embodiment, the rich air-fuel ratio (A/F)RR is kept constant at 13.8 regardless the engine operating condition, and thus the target air-fuel ratio (A/F)T is made 13.8 in step 70. Next, the routine goes to step 72.

Contrarily, if the NO_(x) release flag is reset in step 69, that is, if the lean operation is to be performed in the second group 1b, the routine goes to step 71, where the lean air-fuel ratio (A/F)L is memorized as the target air-fuel ratio (A/F)T. In this embodiment, the lean air-fuel ratio (A/F)L is kept constant at 18.5 regardless the engine operating condition, and thus the target air-fuel ratio (A/F)T is made 18.5 in step 71. Next, the routine goes to step 72.

In step 72, the fuel injection time TAU is calculated using the following equation:

    TAU=TB·((A/F)S/(A/F)T)·FAF

Each fuel injector 5 injects the fuel for the fuel injection time TAU.

In the prior art, there is known an exhaust gas purifying device in which: a NO_(x) -OR catalyst is arranged in the exhaust passage; all of the cylinders of the engine basically perform the lean operation and NO_(x) therefrom is occluded in the NO_(x) -OR catalyst; and the engine temporarily performs the rich operation to thereby release and reduce the occluded NO_(x). However, when the NO_(x) occluding capacity is small due to the occluded NO_(x) amount S(NO_(x)) being large or the deterioration of the NO_(x) -OR catalyst, or when the NO_(x) amount or concentration flowing into the NO_(x) -OR catalyst widely increases, some of the inflowing NO_(x) leaks from the NO_(x) -OR catalyst. The leaked NO_(x) is then emitted to the ambient air in the prior art device.

Contrarily, in this embodiment, NH₃ is synthesized from NO_(x) from the first group 1a, and is fed to the exhaust passage downstream of the NO_(x) -OR catalyst 11a. Thus, even if NO_(x) is leaking from the NO_(x) -OR catalyst, the NO_(x) is reduced by the NH₃. Namely, the leaking NO_(x) is prevented from being emitted to the ambient air.

Next, another embodiment of the operation change control in the engine shown in FIG. 1 will be explained.

As mentioned above, the excess NH₃ produced when the first and the second cylinder groups 1a and 1b perform the rich and the lean operations, respectively is adsorbed in the NH₃ -AO catalyst 14a. Thus, as a period in which the first and the second cylinder groups 1a and 1b perform the rich and the lean operations becomes longer, the adsorbed NH₃ amount becomes larger. However, if the NH₃ -AO catalyst 14a is saturated with NH₃, NH₃ flows out from the NH₃ -AO catalyst 14a. The NH₃ will be purified in the following NH₃ purifying catalyst 16, but it is preferable that the NH₃ amount flowing out from the NH₃ -AO catalyst 14a is as small as possible. If there is no NH₃ flowing out from the NH₃ -AO catalyst 14a, there is no need for providing the NH₃ purifying catalyst 16.

Therefore, in this embodiment, the operation change control of the first and the second groups 1a and 1b is executed in accordance with the adsorbed NH₃ amount in the NH₃ -AO catalyst 14a. Namely, first, the adsorbed NH₃ amount S(NH₃) in the NH₃ -AO catalyst 14a is found, and when the adsorbed NH₃ amount S(NH₃) becomes larger than a predetermined, upper threshold amount UT(NH₃), the operation in the first group 1a is changed from the rich operation to the lean operation and that in the second group 1b is changed from the lean operation to the rich operation. When the adsorbed NH₃ amount S(NH₃) becomes smaller than a predetermined, lower threshold amount LT(NH₃), the operation in the first group 1a is changed from the lean operation to the rich operation and that in the second group 1b is changed from the rich operation to the lean operation.

FIG. 12 shows a time chart illustrating the adsorbed NH₃ amount S(NH₃) in the NH₃ -AO catalyst 14a, and the target air-fuel ratios (A/F)T for the first and the second groups 1a and 1b. In FIG. 12, the time zero represents a time when the first and the second groups 1a and 1b start the rich and the lean operations, respectively. When the first group 1a performs the rich operation with the target air-fuel ratio (A/F)T being the rich air-fuel ratio (A/F)R, and the second group 1b performs the lean operation with the target air-fuel ratio (A/F)T being the lean air-fuel ratio (A/F)L, the adsorbed NH₃ amount S(NH₃) becomes larger, and is larger than the upper threshold amount UT(NH₃) at the time c. When S(NH₃)>UT(NH₃), the target air-fuel ratio (A/F)T for the first group 1a is set to the lean air-fuel ratio (A/F)LL, and that for the second group 1b is set to the rich air-fuel ratio (A/F)RR. As a result, the adsorbed NH₃ is desorbed and the adsorbed NH₃ amount S(NH₃) becomes smaller. At the time d, the adsorbed NH₃ amount S(NH₃) is smaller than the lower threshold LT(NH₃), and the target air-fuel ratios (A/F)T for the first and the second groups 1a and 1b are set again to the rich and the lean air-fuel ratio (A/F)R and (A/F)L, respectively.

It is difficult to directly find the adsorbed NH₃ amount S(NH₃) in the NH₃ -AO catalyst 14a. Therefore, in this embodiment, the adsorbed NH₃ amount S(NH₃) is estimated on the basis of the NH₃ amount synthesized in the TW catalyst Ba or flowing into the NH₃ -AO catalyst 14a, and of the NO_(x) amount passing through the NO_(x) -OR catalyst 11a or flowing into the NH₃ -AO catalyst 14a.

A sensor for detecting the NH₃ amount flowing into the NH₃ -AO catalyst 14a may be arranged in the interconnecting duct 13 between the TW catalyst 8a and the NH₃ -AO catalyst 14a. However, the synthesized NH₃ amount can be estimated on the basis of the NO_(x) amount flowing into the TW catalyst 8a, and the NO_(x) amount flowing into the TW catalyst 8a can be estimated on the basis of the engine operating condition. That is, the synthesized NH₃ amount per unit time becomes larger as the NO_(x) amount flowing into the TW catalyst 8a per unit time becomes larger. Also, the synthesized NH₃ amount per unit time becomes larger as the NH₃ synthesizing efficiency ETA becomes higher.

On the other hand, the NO_(x) amount exhausted from the first group 1a per unit time becomes larger as the engine speed N becomes higher, and thus the NO_(x) amount Qa(NO_(x)) flowing into the TW catalyst 8a per unit time becomes larger. Also, the exhaust gas amount exhausted from the first group 1a becomes larger and the combustion temperature becomes higher as the engine load Q/N becomes higher, and thus the NO_(x) amount flowing into the TW catalyst 8a per unit becomes larger as the engine load Q/N becomes higher.

FIG. 13A illustrates the relationships, obtained by experiment, between the NO_(x) amount exhausted from the first group 1a per unit time Qa(NO_(x)), the engine load Q/N, and the engine speed N, with the constant rich air-fuel ratio (A/F)R. In FIG. 13A, the curves show the identical NO_(x) amounts. As shown in FIG. 13A, the exhausted NO_(x) amount Qa(NO_(x)) becomes larger as the engine load Q/N becomes higher, and as the engine speed N becomes higher. Note that the exhausted NO_(x) amount Qa(NO_(x)) is stored in the ROM 22 in advance in the form of a map as shown in FIG. 13B.

The NH₃ synthesizing efficiency ETA varies in accordance with the temperature TTC of the exhaust gas flowing into the Tw catalyst 8a, which represents the temperature of the TW catalyst 8a. That is, as shown in FIG. 14, the synthesizing efficiency ETA becomes higher as the exhaust gas temperature TTC becomes higher when TTC is low, and becomes lower as TTC becomes higher when TTC is high, with a constant rich air-fuel ratio (A/F)R. The synthesizing efficiency ETA is stored in the ROM 22 in advance in the form of a map as shown in FIG. 14.

Note that the exhausted NO_(x) amount from the first group 1a per unit time Qa(NO_(x)) varies in accordance with the engine air-fuel ratio of the first group 1a. Therefore, if the rich air-fuel ratio (A/F)R is changed in accordance with, for example, the engine operating condition, the exhausted NO_(x) amount Qa(NO_(x)) obtained by the map shown in FIG. 13B must be corrected on the basis of the actual rich air-fuel ratio (A/F)R. Further, the synthesizing efficiency ETA also varies in accordance with the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a, that is, the rich air-fuel ratio (A/F)R, as shown in FIG. 2A. Therefore, if the rich air-fuel ratio (A/F)R is changed in accordance with, for example, the engine operating condition, the synthesizing efficiency ETA obtained by the map shown in FIG. 14 also must be corrected on the basis of the actual rich air-fuel ratio (A/F)R. Or, the efficiency ETA must be obtained by using a map representing a relationship between the efficiency ETA and the rich air-fuel ratio (A/F)R.

The product of Qa(NO_(x)) calculated using the engine load Q/N and the engine speed N and the synthesizing efficiency ETA calculated using the exhaust gas temperature TTC represents the NH₃ amount F(NH₃) flowing into the NH₃ -AO catalyst 14a per unit time.

Note that the exhaust gas temperature TTC is determined in accordance with the engine operating condition such as the engine load Q/N and the engine speed N, and thus the synthesizing efficiency ETA is also determined in accordance with the engine load Q/N and the engine speed N. Accordingly, both Qa(NO_(x)) and ETA are determined in accordance with the engine load Q/N and the engine speed N. Therefore, the synthesized NH₃ amount in the TW catalyst 8a per unit time may be stored in advance in the form of a map, as a function of the engine operating condition such as the engine load Q/N and the engine speed N, and the inflowing NH₃ amount F(NH₃) may be calculated by using the map.

The NO_(x) amount F(NO_(x)) passing through the NO_(x) -OR catalyst 11a and flowing into the NH₃ -AO catalyst 14a per unit time when the second group 1b performs the lean operation is calculated by using the map shown in FIG. 7B.

Further, the NO_(x) amount Qb(NO_(x)) flowing into the NO_(x) -OR catalyst 11a per unit time is calculated by using the map shown in FIG. 6B. Further, the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a is calculated by the method in the above-mentioned embodiment.

If KC represents an NH₃ amount required for reducing unit inflowing NO_(x) amount in the NH₃ -AO catalyst 14a, KC·F(NO_(x)) represents an NH₃ amount consumed by the NO_(x) reduction when NO_(x) flows into the NH₃ -AO catalyst 14a by F(NO_(x)) per unit time. Thus, the excess NH₃ amount per unit time, that is, the NH₃ amount adsorbed in the NH₃ -AO catalyst 14a per unit time is expressed by F(NH₃)-KC·F(NO_(x)).

Accordingly, when the first and the second groups 1a and 1b perform the rich and the lean operation, the adsorbed NH₃ amount S(NH₃) in the NH₃ -AO catalyst 14a is calculated using the following equation:

    S(NH.sub.3)=S(NH.sub.3)+{F(NH.sub.3)-KC·F(NO.sub.x)}·DELTAaa

where DELTAaa represents the time interval of the detection of F(NH₃) and F(NO_(x)). Thus, {F(NH₃)-KC·F(NO_(x))}·DELTAaa represents the NH₃ amount adsorbed in the NH₃ -AO catalyst 14a from the last the detection of F(NH₃) and F(NO_(x)) until the present detection.

KC is a coefficient determined in accordance with the components of the NO_(x) flowing into the NH₃ -AO catalyst 14a, that is, the fractions of NO and NO₂ with respect to the total inflowing NO_(x), and is referred as an equivalent coefficient. The equivalent coefficient KC is set to 4/3 when all of the NO_(x) flowing into the NH₃ -AO catalyst 14a is NO₂, as can be understood from the above-mentioned reaction (9), and is set to 1 when all of the NO_(x) is NO, as can be understood from the above-mentioned reaction (10). The fractions of NO and NO₂ are determined in accordance with the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a and the exhaust gas temperature TAC. Thus, when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a is kept constant, the coefficient KC is determined in accordance with TAC. FIG. 15 illustrates the relationship. As shown in FIG. 15, the equivalent coefficient KC becomes larger as the exhaust gas temperature TAC becomes higher when TAC is low, and becomes smaller as TAC becomes higher when TAC is high, and is kept 1 when TAC becomes further higher. The equivalent coefficient KC is stored in the ROM 22 in advance in the form of a map as shown in FIG. 15. Note that F(NH₃)/KC represents a NO_(x) amount which can be purified by the NH₃ when the NH₃ flows into the NH₃ -AO catalyst 14a by F(NH₃).

On the other hand, FIG. 16A illustrates the NH₃ amount D(NH₃) desorbed from the NH₃ -AO catalyst 14a per unit time, when the first and the second groups 1a and 1b perform the lean and the. rich operations, respectively, obtained by experiment. In FIG. 16A, the curves show the identical desorbed NH₃ amounts. As shown in FIG. 16A, the desorbed NH₃ amount D(NH₃) becomes larger as the adsorbed NH₃ amount S(NH₃) becomes larger. Also, D(NH₃) becomes larger as the temperature TAC becomes higher. The desorbed NH₃ amount D(NH₃) is stored in the ROM 22 in advance in the form of a map as shown in FIG. 16B.

Accordingly, when the first and the second group 1a and 1b perform the lean and the rich operations, respectively, the adsorbed NH₃ amount S(NH₃) is calculated using the following equation:

    S(NH.sub.3)=S(NH.sub.3)-D(NH.sub.3)·DELTAad

where DELTAad represents the time interval of the detection of D(NH₃), and thus D(NH₃)·DELTAad represents the NH₃ amount desorbed from the NH₃ -AO catalyst 14a, from the last detection of D(NH₃) until the present detection.

To obtain the temperature TTC of the exhaust gas flowing into the TW catalyst 8a, and the temperature TAC of the exhaust gas flowing into the NH₃ -AO catalyst 14a, temperature sensors may be arranged in the exhaust passage directly upstream of the TW catalyst 8a and directly upstream of the NH₃ -AO catalyst 14a, respectively. However, the exhaust gas temperatures can be estimated on the basis of the engine operating condition, that is, the engine load Q/N and the engine speed N. Thus, in the embodiment, TTC and TAC are stored in the ROM 22 in advance in the form of a map as shown in FIGS. 17 and 18. ETA and D(NH₃) are calculated using TTC and TAC obtained by the maps shown in FIGS. 17 and 18.

FIGS. 19 and 20 illustrate a routine for executing the second embodiment mentioned above. The routine is executed by interruption every predetermined time.

Referring to FIG. 19, first, in step 80, the exhaust gas temperature TAC is calculated using the map shown in FIG. 18. In the following step 81, the occluded NO_(x) amount S(NO_(x)) is calculated. In the following step 82, it is judged whether an NH₃ desorption flag is set. The NH₃ desorption flag is set when the lean and the rich operations are to be performed in the first and the second groups 1a and 1b, respectively, to desorb NH₃ from the NH₃ -AO catalyst 14a, and is reset when the rich and the lean operations are to be performed in the first and the second groups 1a and 1b, respectively. If the NH₃ desorption flag is reset, the routine goes to step 83.

The steps 83 to 86 are for calculating the inflowing NH₃ amount F(NH₃). In step 83, the exhausted NO_(x) amount Qa(NO_(x)) is calculated using the map shown in FIG. 13B. In the following step 84, the exhaust gas temperature TTC is calculated using the map shown in FIG. 17. In the following step 85, the NH₃ synthesizing efficiency ETA is calculated using the map shown in FIG. 14. In the following step 86, the inflowing NH₃ amount F(NH₃) is calculated using the following equation:

    F(NH.sub.3)=QA(NO.sub.x)·ETA

The following steps 87 and 88 are for calculating the inflowing NO_(x) amount F(NO_(x)). In step 87, the exhausted NO_(x) amount Qb(NO_(x)) is calculated using the map shown in FIG. 6B. In the following step 88, the inflowing NO_(x) amount F(NO_(x)) is calculated using the map shown in FIG. 7B. In the following step 89, the equivalent coefficient KC is calculated using the map shown in FIG. 15.

In the following step 90, the adsorbed NH₃ amount S(NH₃) is calculated using the following equation:

    S(NH.sub.3)=S(NH.sub.3)+{F(NH.sub.3)-KC·F(NO.sub.x)}·DELTAaa

where DELTAaa is a time interval from the last processing cycle until the present processing cycle. In the following step 91, it is judged whether the adsorbed NH₃ amount S(NH₃) is larger than the upper threshold amount UT(NH₃). If S(NH₃)≦UT(NH₃), the processing cycle is ended. Namely, if S(NH₃)≦UT(NH₃), the NH₃ adsorbing capacity of the NH₃ -AO catalyst 14a is judged to be still large, and thus the first and the second groups 1a and 1b continuously perform the rich and the lean operations.

If S(NH₃)>UT(NH₃) in step 91, the routine goes to step 92, where the NH₃ desorption flag is set, and then the processing cycle is ended. Namely, if S(NH₃)>UT(NH₃), the NH₃ adsorbing capacity of the NH₃ -AO catalyst 14a is judged to become small. Thus, the first group 1a stops the rich operation and starts the lean operation, and the second group 1b stops the lean operation and starts the rich operation.

When the NH₃ desorption flag is set, the routine goes from step 82 to step 93, where the desorbed NH₃ amount D(NH₃) is calculated using the map shown in FIG. 16B. In the following step 94, the adsorbed NH₃ amount S(NH₃) is calculated using the following equation:

    S(NH.sub.3)=S(NH.sub.3)-D(NH.sub.3)·DELTAad

where DELTAad is a time interval from the last processing cycle until the present processing cycle. In the following step 95, it is judged whether the adsorbed NH₃ amount S(NH₃) is smaller than the lower threshold amount LT (NH₃). If S(NH₃)≧LT(NH₃), the processing cycle is ended. Namely, if S(NH₃)≧LT(NH₃), the NH₃ adsorbing capacity is judged to be still small, and thus the first and the second groups 1a and 1b continuously performs the lean and the rich operations.

If S(NH₃)<LT(NH₃), the routine goes to step 96, where the NH₃ desorption flag is reset and the processing cycle is ended. Namely, if S(NH₃)<LT(NH₃), the NH₃ adsorbing capacity is judged to be large. Thus, the first group 1a stops the lean operation and starts the rich operation, and the second group 1b stops the rich operation and starts the lean operation.

FIG. 20 illustrates a portion corresponding to the step 81 shown in FIG. 19.

Referring to FIG. 20, first, in step 100, it is judged whether the NH₃ desorption flag, which is set or reset in the routine shown in FIG. 19, is set. If the NH₃ desorption flag is reset, that is, if the second group 1b performs the lean operation, the routine goes to step 101, where Qb(NO_(x)) is calculated using the map shown in FIG. 6B. In the following step 102, the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a is calculated using the following equation:

    S(NO.sub.x)=S(NO.sub.x)+{Qb(NO.sub.x)-F(NO.sub.x)}·DELTAna

where DELTAna is a time interval from the last processing cycle until the present processing cycle. Then, the processing cycle is ended.

Contrarily, if the NH₃ desorption flag is set, that is, if the second group 1b performs the lean rich operation, the routine goes from step 100 to step 103, where the exhaust gas temperature TNC is calculated using the map shown in FIG. 9. In the following step 104, the released NO_(x) amount D(NO_(x)) is calculated using the map shown in FIG. 8B. In the following step 105, the occluded NO_(x) amount S(NO_(x)) is calculated using the following equation:

    S(NO.sub.x)=S(NO.sub.x)-D(NO.sub.x)·DELTAnd

where DELTAnd is a time interval from the last processing cycle until the present processing cycle.

Note that, alternatively, both the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a and the adsorbed NH₃ amount S(NH₃) in the NH₃ -OR catalyst 14a may be found, and the operation change control of the groups 1a and 1b may be executed when at least one of S(NO_(x)) and S(NH₃) becomes larger than the corresponding upper threshold, or smaller than the corresponding lower threshold.

The occlusive material 11 in the embodiments described the above comprises the NO_(x) occluding and releasing function. However, the NO_(x) releasing function of the occlusive material 11 may be omitted. In this case, the occluding material 11 may be replaced with a new one when the occluded NO_(x) amount becomes large, to thereby continuously keep the NO_(x) amount flowing into the NH₃ -AO catalyst 14a small.

Next, another embodiment of the exhaust gas purifying method in the engine shown in FIG. 1 will be explained.

In the above embodiments, the rich air-fuel ratio (A/F)R and the lean air-fuel ratio (A/F)LL for the first group 1a, and the lean air-fuel ratio (A/F)L and the rich air-fuel ratio (A/F)RR for the second group 1b are set to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a lean, both when the first and the second groups 1a and 1b respectively perform the rich and the lean operations, and when the first and the second groups 1a and 1b respectively perform the lean and the rich operations. Namely, in the above embodiments, the rich air-fuel ratios (A/F)R and (A/F)RR are both set to about 13.8, and the lean air-fuel ratios (A/F)L and (A/F)LL are both set to about 18.5.

As mentioned above, just after the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a changes from lean to rich, NO_(x) may escape from the catalyst 11a without being purified, due to the lack of the reducing agent. However, if the escaping NO_(x) amount is large, such a large amount of the escaping NO_(x) may not be purified sufficiently on the following NH₃ -AO catalyst 14a. Thus, it is preferable that the escaping NO_(x) amount is as small as possible.

Therefore, in this embodiment, when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a is to be made rich, the exhaust gas air-fuel ratio is made smaller or richer than that in the above-mentioned embodiments, to decrease the escaping NO_(x) amount. That is, the rich air-fuel ratio (A/F)RR with which the second group 1b performs the rich operation is set to about 12.5, for example. The smaller or richer rich air-fuel ratio (A/F)RR increases the amount of the reducing agent flowing into the NH₃ -AO catalyst 14a, to thereby decrease the escaping NO_(x) amount.

As mentioned above, the inflowing NO_(x) is converted to NH₃ in the NO_(x) -OR catalyst 11a. The NH₃ reduces NO_(x), on the NO_(x) -OR catalyst 11a, and thus it is preferable that the NH₃ amount synthesized in the NO_(x) -OR catalyst 11a is made larger, to decrease the escaping NO_(x) amount. Thus, alternatively, when the second group 1b has to perform the rich operation, some of the cylinders of the second group 1b may perform the rich operation with the rich air-fuel ratio of about 12.5, and the remaining may perform the rich operation with the rich air-fuel ratio of about 13.8. This also results in suppressing the deterioration of the fuel consumption rate.

In the above-mentioned embodiments, the first group 1a performs the lean operation when the second group 1b performs the rich operation. However, if the adsorbed NH₃ amount in the NH₃ -AO catalyst 14a is small when the second group 1b starts the rich operation, the escaping NO₃ may not be purified sufficiently on the NH₃ -AO catalyst 14a. Thus, in this embodiment, the first group 1a performs the rich operation continuously to synthesize NH₃ in the TW catalyst 8a, even when the second group 1b performs the rich operation, to thereby supply NH₃ to the NH₃ -AO catalyst 14a continuously. As a result, NO_(x) is purified sufficiently on the NH₃ -AO catalyst 14a, regardless whether when the NO_(x) -OR catalyst 11a occludes NO_(x) therein or when the catalyst 11a releases NO_(x) therefrom. Note that, in this embodiment, the rich air-fuel ratio (A/F)R for the first group 1a is kept about 13.8, regardless whether the second group 1b performs the lean or the rich operation.

In this way, if the rich air-fuel ratio (A/F)RR for the second group 1b is made smaller or richer and the first group 1a continuously performs the rich operation, the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a is made rich, and this prevents good purification of NO_(x) and NH₃. Thus, when the second group 1b has to perform the rich operation, the secondary air is supplied to the NH₃ -AO catalyst 14a by the secondary air supplying device 18, to thereby keep the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a lean. Note that when the second group 1b performs the lean operation, the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a is kept lean, even without the secondary air.

In this embodiment, the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a is found when the first and the second groups 1a and 1b respectively perform the rich and the lean operations, and when the occluded NO_(x) amount S(NO_(x)) is larger than the upper threshold amount UT(NO_(x)), the operation in the second group 1b is changed to the rich operation, while the operation in the first group la is kept as the rich operation. The operation in the second group 1b is returned to the lean operation when a rich period has past since the second group 1b started the rich operation. The rich period is a period required to make the occluded NO_(x) amount S(NO_(x)) equal to zero, for example, and is predetermined as a function of the engine operating condition such as the engine load Q/N and the engine speed N. Alternatively, the rich period may be set to a period required to make the occluded NO_(x) amount S(NO_(x)) equal to the lower threshold LT(NO_(x)) mentioned above. Further, alternatively, the rich period may be set as a function of the engine operation and the exhaust gas temperature TNC.

FIG. 21 shows a time chart illustrating the actual occluded NO_(x) amount in the NO_(x) -OR catalyst 11a, a counter value COUNT, the target air-fuel ratios (A/F)T for the first and the second groups 1a and 1b, the operation of the secondary air supplying device 18, and the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a. In FIG. 21, the time zero represents a time when the second group 1b starts the rich operation. Just after time a, the target air-fuel ratios (A/F)T of the first and the second groups 1a and 1b are the rich and the lean air-fuel ratios (A/F)R and (A/F)L, respectively, and the supply of the secondary air by the device 18 is stopped (OFF).

At the time a1, the occluded NO_(x) amount is larger than the upper threshold amount UT(NO_(x)), and the target air-fuel ratio (A/F)T for the second group 1b is set to the rich air-fuel ratio (A/F)RR, while that for the first group 1a is kept as the rich air-fuel ratio (A/F)R. At the same time, the secondary air is supplied (ON). As a result, the occluded NO_(x) is released and the occluded NO_(x) amount becomes smaller. At this time, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a is kept lean, and the exhaust gas is sufficiently purified on the catalyst 14a. Further, at the time a1, the counter value COUNT is set to a rich period value CR. The counter value COUNT represents a period in which the second group 1b performs the rich operation, and the rich period value CR corresponds to the rich period mentioned above. The rich period value CR is obtained in advance by experiment, and is stored in the ROM 22 in the form of a map as shown in FIG. 22, as a function of the engine load Q/N and the engine speed N.

The counter value COUNT is decremented from the rich period value CR by 1. When the counter value COUNT becomes zero at the time b1, the actual occluded NO_(x) amount becomes substantially zero. At this time, the target air-fuel ratio (A/F)T for the second group 1b is set again to the lean air-fuel ratio (A/F)L. Further, the supply of the secondary air is stopped at this time.

As shown in FIG. 21, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a when the second group 1b performs the rich operation is smaller or closer to the stoichiometric than that when the second group 1b performs the lean operation. To ensure good purification of the exhaust gas on the NH₃ -AO catalyst 14a, the exhaust gas air-fuel ratio of the inflowing exhaust gas is merely made lean, and the exhaust gas air-fuel ratio is unnecessarily kept constant. Rather, if the exhaust gas air-fuel ratio is kept constant, a considerably large amount of the secondary air is required. Such a large amount of the secondary air drops the temperature of the NH₃ -AO catalyst 14a, and thereby good purification may be hindered. Thus, in this embodiment, the secondary air amount is made the minimum amount required to keep the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a lean. Note that the minimum amount is found in advance by experiment, and is stored in the ROM 22.

FIG. 23 illustrates a routine for executing the operation change control in the second group 1b, according to the embodiment. The routine is executed by interruption every predetermined time.

Referring to FIG. 23, first, in step 400, it is judged whether a NO_(x) release flag is set. The NO_(x) release flag is set when the rich operation is to be performed in the second group 1b to release NO_(x) from the NO_(x) -OR catalyst 11a, and is reset when the lean operation is to be performed in the second group 1b. If the NO_(x) release flag is reset, the routine goes to step 401, where Qb(NO_(x)) is calculated using the map shown in FIG. 6B. In the following step 402, F(NO_(x)) is calculated using the map shown in FIG. 7B. In the following step 403, the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a is calculated using the following equation:

    S(NO.sub.x)=S(NO.sub.x)+{Qb(NO.sub.x)-F(NO.sub.x)}·DELTAna

where DELTAna is a time interval from the last processing cycle until the present processing cycle. In the following step 404, it is judged whether the occluded NO_(x) amount S(NO_(x)) is larger than the upper threshold amount UT(NO_(x)). If S(NO_(x))≦UT(NO_(x)), the processing cycle is ended. Namely, if S(NO)≦UT(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to be still large, and thus the second group 1b continuously performs the lean operation.

If S(NO_(x))>UT(NO_(x)) in step 404, the routine goes to step 405, where the NO_(x) release flag is set, and then the processing cycle is ended. Namely, if S(NO_(x))>UT(NO_(x)), the NO_(x) occluding capacity is judged to become small. Thus, the second group 1b stops the lean operation and starts the rich operation. In the following step 406, the supply of the secondary air starts. In the following step 407, the occluded NO:, amount S(NO_(x)) is reset. Then, the processing cycle is ended.

Contrarily, if the NO_(x) release flag is set, the routine goes from step 400 to step 408, where a calculation flag is set. The calculation flag is set when the rich period value CR is calculated, and is reset when the counter value COUNT is made zero. When it is first time for the routine to go to step 408 after the NO_(x) release flag is set, the calculation flag is reset, and thus the routine goes to step 409, where the rich period value CR is calculated using the map shown in FIG. 22. In the following step 410, the rich period value CR is memorized as COUNT. In the following step 411, the calculation flag is set. Then, the processing cycle is ended.

When the calculation flag is set, the routine goes from step 408 to step 412, where the counter value COUNT is decremented by 1. In the following step 413, it is judged whether the counter value COUNT is zero. If COUNT is larger than zero, the processing cycle is ended. Namely, if COUNT>0, the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to be still small, and thus the rich operation of the second group 1b and the supply of the secondary air are continued.

If COUNT=0 in step 413, the routine goes to step 414, where the NO_(x) release flag is reset. Namely, if COUNT=0, the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to become sufficiently large, and thus the second group 1b stops the rich operation and starts the lean operation. In the following step 415, the supply of the secondary air is stopped. In the following step 416, the calculation flag is reset, and then the processing cycle is ended.

Next, further another embodiment of the exhaust gas purifying method of the engine shown in FIG. 1 will be explained.

In the embodiments mentioned above, the occlusive material 11 is arranged in the exhaust passage between the second group 1b and the exhaust gas purifying catalyst 14, to thereby prevent the NO_(x) amount flowing into the exhaust gas purifying catalyst 14 from exceeding a NO_(x) amount which can be reduced by the inflowing NH₃. On the other hand, the NO_(x) amount flowing into the exhaust gas purifying catalyst 14 becomes smaller as the NO_(x) amount exhausted from the second group 1b. Thus, in this embodiment, the NO_(x) amount exhausted from the second group 1b is decreased to thereby prevent the NO_(x) amount flowing into the exhaust gas purifying catalyst 14 from exceeding a NO_(x) amount which can be reduced by the inflowing NH₃.

As mentioned above, the NO_(x) amount exhausted from the second group 1b varies in accordance with the engine air-fuel ratio. Thus, in this embodiment, the lean air-fuel ratio (A/F)L with which the second group 1b performs the lean operation is controlled, to thereby prevent the NO_(x) amount flowing into the exhaust gas purifying catalyst 14 from exceeding a NO_(x) amount which can be reduced by the inflowing NH₃.

FIG. 24 illustrates the experimental results showing the NO_(x) amount exhausted from the second group 1b per unit time, at the respective engine air-fuel ratio, and under a constant engine operating condition. In the example shown in FIG. 24, the exhausted NO_(x) amount becomes maximum at the engine air-fuel ratio being about 17.5, and becomes smaller as the engine air-fuel ratio becomes richer or leaner with respect to 17.5. Further, as can be seen from FIG. 24, the exhausted NO_(x) amount at the engine air-fuel ratio being 18.5 is substantially identical to that at the engine air-fuel ratio being (A/F)N. Thus, when the engine air-fuel ratio is made 18.5, if the engine air-fuel ratio is made larger than 18.5, or is set to a lean air-fuel ratio which is smaller than (A/F)N or to stoichiometric, the NO_(x) amount exhausted from the second group 1b can be decreased. Note that (A/F)N is about 16.5 in the example shown in FIG. 24.

Therefore, in this embodiment, if the NO_(x) amount flowing into the exhaust gas purifying catalyst 14 exceeds a NO_(x) amount which can be reduced by the inflowing NH₃ when the second group 1b performs the lean operation with the lean air-fuel ratio (A/F)L being 18.5, the lean air-fuel ratio (A/F)L is made larger or leaner than 18.5, such as 25.0, to thereby prevent the NO_(x) amount flowing into the exhaust gas purifying catalyst 14 from exceeding a NO_(x) amount which can be reduced by the inflowing NH₃. Alternatively, the lean air-fuel ratio (A/F)L may be changed to an air-fuel ratio which is larger or leaner than the stoichiometric air-fuel ratio (A/F)S and is smaller or richer than (A/F)N, or to the stoichiometric air-fuel ratio (A/F)S, to thereby ensure a large output torque from the engine.

The detailed explanation of the embodiment will be made with reference to FIG. 25. The routine shown in FIG. 25 is executed by interruption every predetermined time.

Referring to FIG. 25, first, in step 110, it is judged whether a rich flag is set. The rich flag is set when the second group 1b has to perform the rich operation and is reset when the second group 1b has to perform the lean operation. As the rich flag, the NO_(x) release flag in the routine shown in FIG. 10, or the NH₃ desorption flag in the routine shown in FIG. 19 can be used. If the rich flag is set, that is, if the second group 1b has to perform the rich operation, the routine jumps to step 116.

If the rich flag is reset, that is, if the second group 1b has to perform the lean operation, the routine goes to step 111, where the inflowing NH₃ amount F(NH₃) into the NH₃ -AO catalyst 14a is calculated. In this step 111, the steps 83 to 86 in the routine shown in FIG. 19 are performed, for example. In the following step 112, the inflowing NO_(x) amount F(NO_(x)) into the NH₃ -AO catalyst 14a is calculated. In this step 112, the steps 87 and 88 in the routine shown in FIG. 19 are performed, for example. In the following step 113, the equivalent coefficient KC is calculated. In this step 113, the steps 80 and 89 in the routine shown in FIG. 19 are performed, for example.

In the following step 114, it is judged whether F(NH₃) is larger than F(NO_(x))·KC. If F(NH₃)<F(NO_(x))·KC, that is, if the inflowing NO_(x) amount is larger than a NO_(x) amount which can be reduced by the inflowing NH₃, the routine goes to step 115, where the lean air-fuel ratio (A/F)L for the second group 1b is changed to 25.0. Contrarily, if F(NH₃)≧F(NO_(x))·KC, that is, if the inflowing NO_(x) amount is equal to or smaller than a NO_(x) amount which can be reduced by the inflowing NH₃, the routine goes from step 114 to step 116, where the lean air-fuel ratio (A/F)L for the second group 1b is kept 18.5. Then, the processing cycle is ended.

In step 115, the lean air-fuel ratio (A/F)L is made equal to 25.0, which is larger than 18.5. Alternatively, (A/F)N, which may vary in accordance with the engine operating condition, is found in advance, and the lean air-fuel ratio (A/F)L may be changed to an air-fuel ratio which is larger than (A/F)S and is smaller than (A/F)N, or to the stoichiometric air-fuel ratio (A/F)S.

If the NO_(x) amount flowing into the NH₃ -AO catalyst 14a is decreased by decreasing the NO_(x) amount exhausted from the second group 1b, as mentioned above, a period during which the second group 1b has to perform the lean operation can be extended, and thus the fuel consumption rate is further lowered. Further, the frequency of the operation change in the second group 1b is decreased, and thus the fluctuation in the output torque of the engine 1 is diminished, to thereby enhance the drivability. Further, the volume of the occlusive material 11 can be decreased, or the occlusive material 11 can be omitted. When the occlusive material 11, such as the NO_(x) -OR catalyst 11a is omitted, there is no need for the second group 1b to perform the rich operation, and thus the fuel consumption rate is further lowered.

Next, another embodiment for decreasing the NO_(x) amount exhausted from the second group 1b will be explained.

To decrease the NO_(x) amount exhausted from the second group 1b, in this embodiment, the operation of at least one of the cylinders of the second group 1b is stopped temporarily. Namely, the number of the operating cylinder in the second group 1b is decreased to thereby decrease the NO_(x) amount flowing into the NH₃ -AO catalyst 14a.

The detailed explanation for the embodiment will be made with reference to FIG. 26. The routine shown in FIG. 26 is executed by interruption every predetermined time. Also, in FIG. 26, steps 120 to 124 correspond to the steps 110 to 114, respectively, and the explanation therefor is omitted.

Referring to FIG. 26, if F(NH₃)<KC·F(NO_(x)) in step 124, that is, if the inflowing NO_(x) amount is larger than a NO_(x) amount which can be reduced by the inflowing NH₃, the routine goes to step ¹²⁵, where the number of the cylinder to be operating in the cylinders of the second group 1b is decremented by, for example, 1. Namely, the number of the cylinder to be stopped is incremented by 1. Then, the processing cycle is ended. If F(NH₃)≧KC·F(NO_(x)) in step 124, that is, if the inflowing NO_(x) amount is smaller than a NO_(x) amount which can be reduced by the inflowing NH₃, the routine goes to step 126, where the number of the cylinder to be operating in the cylinders of the second group 1b is incremented by, for example, 1. Namely, the number of the cylinder to be stopped is decremented by 1. Then, the processing cycle is ended.

In this embodiment, it is preferable that the intake or exhaust valves of the cylinder to be stopped are kept closed during the stoppage thereof, to prevent the intake air from flowing into the exhaust manifold 10. If air which does not contribute to the combustion flows into the exhaust manifold 10, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 14a will deviate from the engine air-fuel ratio of the second group 1b. Additionally, it is preferable that the cylinder to be stopped is not fixed but is changed cyclicly.

In the embodiments explained with reference to FIGS. 25 and 26, the NH₃ amount F(NH₃) flowing into the NH₃ -AO catalyst 14a is obtained, and then the lean air-fuel ratio (A/F)L or the operating cylinder number of the second group 1b is controlled in accordance with F(NH₃). Alternatively, the lean air-fuel ratio or the operating cylinder number suitable for the respective F(NH₃) may be obtained in advance by experiment, and the lean air-fuel ratio or the operating cylinder number may be made equal to the suitable ratio or number.

FIG. 27 shows another embodiment for the exhaust gas purifying device according to the present invention. In FIG. 27, constituent elements the same as those in the above mentioned embodiments are given the same reference numerals. The engine is provided with an intake passage, fuel injectors, air-fuel ratio sensors, and an electronic control unit same as shown in FIG. 1, but they are not depicted in FIG. 27.

Referring to FIG. 27, the engine 1 has eight cylinders. The first, the third, the fifth, and the seventh cylinders #1, #3, #5, and #7 are aligned in one side of the crank shaft (not shown), and the second, the fourth, the sixth, and the eighth cylinders #2, #4, #6, and #8 are aligned in the other side of the crank shaft. The first cylinder #1, which constitutes the first cylinder group 1a, is connected, via the exhaust duct 7, to the catalytic converter housing the TW catalyst 8a therein. In the second to the eighth cylinders #2 to #8, which constitute the second cylinder group 1b, the third, the fifth, and the seventh cylinders #3, #5, and #7, which constitute a first cylinder subgroup 1ba, are connected, via an exhaust manifold 180a, to a catalytic converter 182a housing a TW catalyst 181a therein. Also, the second, the fourth, the sixth, and the eighth cylinders #2, #4, #6, and #8, which constitute a second cylinder subgroup 1bb, are connected to, via an exhaust manifold 180b, a catalytic converter 182b housing a TW catalyst 181b therein. The converters 182a and 182b are connected, via an interconnecting duct 186, to the catalytic converter 12 housing the NO_(x) -OR catalyst 11a therein. Namely, in this embodiment, the TW catalysts are arranged between the second group 1b and the NO_(x) -OR catalyst 11a. Note that, alternatively, the first and the second subgroups 1ba and 1bb may be constituted by at least one cylinder, respectively.

In this engine again, the first group 1a performs the rich operation continuously, and the second group 1b basically performs the lean operation and temporarily performs the rich operation. When the second group 1b performs the lean operation and the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalysts 181a and 181b are made lean, the majority of the inflowing NO is oxidized to NO₂ on the TW catalysts 181a and 181b. Thus, most of the NO_(x) flowing into the NO_(x) -OR catalyst 11a is in the form of NO₂.

As mentioned above, when the NO_(x) is occluded in the NO_(x) -OR catalyst 11a, first, NO₂ is converted to NO₃ ⁻ and then occluded. Thus, the inflowing NO is first oxidized to NO₂ on the NO_(x) -OR catalyst 11a, and then is occluded. In this embodiment, the majority of the inflowing NO_(x) is in the form of NO₂, as mentioned above. Thus, the oxidation of NO is unnecessary, and thereby the inflowing NO_(x) is quickly occluded in the NO_(x) -OR catalyst 11a. As a result, the volume of the NO_(x) -OR catalyst 11a can be decreased with respect to the embodiment shown in, for example, FIG. 1. Further, good purification of the exhaust gas is maintained, even though the NO oxidation ability of the catalyst 11a becomes lower.

On the other hand, when the second group 1b performs the rich operation and the exhaust gas air-fuel ratio flowing into the TW catalysts 181a and 181b is made rich, NH₃ is synthesized from a part of the inflowing NO_(x) in the TW catalysts 181a and 181b. The NH₃ then flows into the NO_(x) -OR catalyst 11a.

As mentioned above, the occluded NO_(x) is released from the NO_(x) -OR catalyst 11a when the exhaust gas air-fuel ratio of the inflowing exhaust gas is made rich. A part of the released NO_(x) is reduced by the inflowing HC and CO. However, the inflowing exhaust gas includes NH₃, of which the reducing ability is high, and thus the released NO_(x) can be immediately reduced by NH₃. This makes a period in which the second group 1b has to perform the rich operation shorter than the embodiment shown in FIG. 1, and thus the fuel consumption rate is further lowered. Additionally, the volume of the exhaust gas purifying catalyst 14 is decreased.

Next, the exhaust gas purifying method in the warming-up operation in the engine shown in FIG. 27 will be explained with reference to FIG. 28. The routine shown in FIG. 28 is executed by interruption every predetermined time.

Referring to FIG. 28, first, in step 190, it is judged whether the warming-up operation is in process. The judgement is executed in accordance with the temperature of the cooling water, the engine oil, the NO_(x) -OR catalyst 11a, the NH₃ -AO catalyst 14a, the exhaust gas flowing into the catalysts, or the intake air. If it is judged that the warming-up operation is in process, the routine goes to step 191, where the target air-fuel ratio for all the cylinders is made equal to the stoichiometric air-fuel ratio (A/F)S. That is, all cylinders perform the stoichiometric operation.

In the warming-up operation, the temperature of the catalysts may be lower than the activating temperature thereof, and thus there may be a case where the exhaust gas is not purified sufficiently even if the first and the second groups 1a and 1b respectively perform the rich and the lean operations.

The TW catalysts 8a, 181a, and 181b are arranged next to the corresponding cylinder(s), and thus the temperature of these catalysts are able to rise up to the activating temperature thereof, quickly. Further, a TW catalyst purifies NO_(x), HC, and CO in the inflowing exhaust gas simultaneously and sufficiently, if the exhaust gas air-fuel ratio of the inflowing exhaust gas is made stoichiometric, as shown in FIG. 2. Therefore, in this embodiment, all of the cylinders perform the stoichiometric operation to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalysts 8a, 181a, and 181b stoichiometric, to thereby ensure good purification of the exhaust gas, in the warming-up operation.

Further, when all of the cylinders perform the stoichiometric operation to purify the exhaust gas on the Tw catalysts 8a, 181a, and 181b, the temperatures of the NO_(x) -OR catalyst 11a and the NH₃ -AO catalyst 14a, which are arranged downstream of the TW catalysts, quickly rise up to the activating temperature thereof.

In step 190, if the warming-up operation is not in process, that is, if the warming-up operation is finished, the routine goes to step 192, where the operation change control mentioned above, such as the routine shown in FIG. 10 or 19, is executed.

Generally, a TW catalyst has good thermal durability. Thus, a temperature of a catalyst located downstream of the TW catalyst is prevented from rising excessively. Namely, in the embodiment shown in FIG. 27, the temperatures of the NO_(x) -OR catalyst 11a, the NH₃ -AO catalyst 14a, and the NH₃ purifying catalyst 16 are prevented from rising excessively. This enhances the durabilities of the catalysts. The other constructions of the exhaust purifying device and the operation thereof are the same as those in the embodiment explained with reference to in FIGS. 1 to 11, and thus the explanations therefor are omitted.

FIG. 29 shows further another embodiment for the exhaust gas purifying device according to the present invention. In FIG. 29, constituent elements the same as those in the above mentioned embodiments are given the same reference numerals. The engine is provided with an intake passage, fuel injectors, air-fuel ratio sensors, and an electronic control unit same as shown in FIG. 1, but they are not depicted in FIG. 29.

Referring to FIG. 29, the occlusive material 11 comprises a pair of the NO_(x) -OR catalysts 11a and 11b. Inlets of catalytic converters 12a and 12b housing the corresponding NO_(x) -OR catalysts 11a and 11b therein are connected to the catalytic converters 182a and 182b housing the Tw catalysts 181a and 181b therein, respectively. Outlets of the catalytic converters 12a and 12b are connected, via the interconnecting duct 186, to the catalytic converter 15 housing the NH₃ -AO catalyst 14a therein. Therefore, the exhaust gas of the first subgroup 1ba of the second group 1b flows via the TW catalyst 181a and the NO_(x) -OR catalyst 11a, and that of the second subgroup 1bb of the second group 1b flows via the TW catalyst 181b and the NO_(x) -OR catalyst 11b, into the NH₃ -AO catalyst 14a.

The first group 1a continuously performs the rich operation, with the rich air-fuel ratio (A/F)R being about 13.8.

The first subgroup 1ba of second group 1b basically performs the lean operation with the lean air-fuel ratio (A/F)L being about 18.5. When the NO_(x) amount Sa(NO_(x)) occluded in the NO_(x) -OR catalyst 11a is larger than a predetermined, upper threshold amount UTa(NO_(x)), the first subgroup 1ba performs the rich operation with the rich air-fuel ratio (A/F)RR being about 13.8, to release the occluded NO_(x) from the NO_(x) -OR catalyst 11a. When a predetermined period has past since the first subgroup 1ba starts the rich operation, the first subgroup 1ba resumes the lean operation.

Also, the second subgroup 1bb basically performs the lean operation with the lean air-fuel ratio (A/F)L being about 18.5. When the NO_(x) amount Sb(NO_(x)) occluded in the NO_(x) -OR catalyst 11b is larger than a predetermined, upper threshold amount UTb(NO_(x)), the second subgroup 1bb performs the rich operation with the rich air-fuel ratio (A/F)RR being about 13.8, to release the occluded NO_(x) from the NO_(x) -OR catalyst 11b. When a predetermined period has past since the second subgroup 1bb starts the rich operation, the second subgroup 1bb resumes the lean operation. This is a basic method for controlling the operation change in the engine shown in FIG. 29.

As mentioned above, when the secondary air is supplied to the exhaust passage, the catalyst temperature may drop to thereby deteriorate the purification of the exhaust gas. Further, if the secondary air supplying device 18 is unnecessary, the structure of the exhaust gas purifying device is simplified. However, if the first and the second subgroups 1ba and 1bb perform the rich operation simultaneously without the secondary air supplying device 18, the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a is made rich, which is not desirable. Therefore, in this embodiment, when one of subgroups is performing the rich operation, the other is prohibited from performing the rich operation, and continuously performs the lean operation. In other words, the overlap of the rich operations of the first and the second subgroups 1ba and 1bb is prevented. As long as one of the subgroups 1ba and 1bb performs the lean operation, the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the NH₃ -AO catalyst 14a is kept lean, even if the other performs the rich operation. Accordingly, good purification of the exhaust gas on the NH₃ -AO catalyst 14a is ensured.

FIG. 30 illustrates a routine for executing the operation change control in the first subgroup 1ba, according to the embodiment. The routine is executed by interruption every predetermined time.

Referring to FIG. 30, first, in step 420, it is judged whether a first NO_(x) release flag is set. The first NO_(x) release flag is set when the first subgroup 1ba has to perform the rich operation to release the occluded NO_(x) from the NO_(x) -OR catalyst 11a, and is reset when the first subgroup 1ba has to perform the lean operation. If the first NO_(x) release flag is reset, the routine goes to step 421, where the NO_(x) amount exhausted from the first subgroup 1ba per unit time Qba(NO_(x)) is calculated using a map shown in FIG. 31. In the following step 422, the NO_(x) amount passing through the NO_(x) -OR catalyst 11a per unit time Fa(NO_(x)) is calculated using a map shown in FIG. 32. In the following step 423, the occluded NO_(x) amount Sa(NO_(x)) in the NO_(x) -OR catalyst 11a is calculated using the following equation:

    Sa(NO.sub.x)=Sa(NO.sub.x)+{Qba(NO.sub.x)-Fa(NO.sub.x)}·DELTAna

where DELTAna is a time interval from the last processing cycle until the present processing cycle. In the following step 424, it is judged whether a second NO_(x) release flag is set. The second NO_(x) release flag is set when the second subgroup 1bb has to perform the rich operation to release the occluded NO_(x) from the NO_(x) -OR catalyst 11b, and is reset when the second subgroup 1bb has to perform the lean operation. If the second flag is set, that is, if the second subgroup 1bb has to perform the rich operation, the processing cycle is ended. Namely, the first subgroup 1ba continuously performs the lean operation.

If the second NO_(x) release flag is reset in step 424, the routine goes to step 425, where it is judged whether the occluded NO_(x) amount Sa(NO_(x)) is larger than the upper threshold amount UTa(NO_(x)). If Sa(NO_(x))≦UTa(NO_(x)), the processing cycle is ended. Namely, if Sa(NO_(x))≦UTa(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to be still large, and thus the first subgroup 1ba continuously performs the lean operation.

If Sa(NO_(x))>UTa(NO_(x)) in step 425, the routine goes to step 426, where the first NO_(x) release flag is set, and then the processing cycle is ended. Namely, if Sa(NO_(x))>UTa(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to become small. Thus, the first subgroup 1ba stops the lean operation and starts the rich operation. In the following step 427, the occluded NO_(x) amount Sa(NO_(x)) is reset. Then, the processing cycle is ended.

Contrarily, if the first NO_(x) release flag is set, the routine goes from step 420 to step 428, where a first calculation flag is set. The first calculation flag is set when a rich period value CRa for the first subgroup 1ba is once calculated, and is reset when a counter value COUNTa, which represents a period in which the first subgroup 1ba is performing the rich operation is made zero. When it is first time for the routine to go to step 428 after the first NO_(x) release flag is set, the first calculation flag is reset, and thus the routine goes to step 429, where the rich period value CRa is calculated using the map shown in FIG. 33. In the following step 430, the rich period value CRa is memorized as COUNTa. In the following step 431, the first calculation flag is set. Then, the processing cycle is ended.

When the first calculation flag is set, the routine goes from step 428 to step 432, where the counter value COUNTa is decremented by 1. In the following step 433, it is judged whether the counter value COUNTa is zero. If COUNTa is larger than zero, the processing cycle is ended. Namely, if COUNTa>0, the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to be still small, and thus the rich operation of the first subgroup 1ba is continued.

If COUNTa=0 in step 433, the routine goes to step 434, where the first NO_(x) release flag is reset. Namely, if COUNTa=0, the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to become sufficiently large, and thus the first subgroup 1bs stops the rich operation and starts the lean operation. In the following step 435, the first calculation flag is reset, and then the processing cycle is ended.

FIG. 34 illustrates a routine for executing the operation change control in the second subgroup 1bb, according to the embodiment. The routine is executed by interruption every predetermined time.

Referring to FIG. 34, first, in step 440, it is judged whether a second NO_(x) release flag is set. The second NO_(x) release flag is set when the second subgroup 1bb has to perform the rich operation to release the occluded NO_(x) from the NO_(x) -OR catalyst 11b, and is reset when the second subgroup 1bb has to perform the lean operation. If the second NO_(x) release flag is reset, the routine goes to step 441, where the NO_(x) amount exhausted from the second subgroup 1bb per unit time Qbb(NO_(x)) is calculated using a map shown in FIG. 35. In the following step 442, the NO_(x) amount passing through the NO_(x) -OR catalyst 11b per unit time Fb(NO_(x)) is calculated using a map shown in FIG. 36. In the following step 443, the occluded NO_(x) amount Sb(NO_(x)) in the NO_(x) -OR catalyst 11b is calculated using the following equation:

    Sb(NO.sub.x)=Sb(NO.sub.x)+{Qbb(NO.sub.x)-Fb(NO.sub.x)}·DELTAna

where DELTAna is a time interval from the last processing cycle until the present processing cycle. In the following step 444, it is judged whether the first NO_(x) release flag is set, which is controlled in the routine shown in FIG. 31. If the first NO_(x) release flag is set, that is, if the first subgroup 1ba has to perform the rich operation, the processing cycle is ended. Namely, the second subgroup 1bb continuously performs the lean operation.

If the first NO_(x) release flag is reset in step 444, the routine goes to step 445, where it is judged whether the occluded NO_(x) amount Sb(NO_(x)) is larger than the upper threshold amount UTb(NO_(x)). If Sb(NO_(x))≦UTb(NO_(x)), the processing cycle is ended. Namely, if Sb(NO_(x))≦UTb(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11b is judged to be still large, and thus the second subgroup 1bb continuously performs the lean operation.

If Sb(NO_(x))>UTb(NO_(x)) in step 445, the routine goes to step 446, where the second NO_(x) release flag is set, and then the processing cycle is ended. Namely, if Sb(NO_(x))>UTb(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11b is judged to be small. Thus, the second subgroup 1bb stops the lean operation and starts the rich operation. In the following step 447, the occluded NO_(x) amount Sb(NO_(x)) is reset. Then, the processing cycle is ended.

Contrarily, if the second NO_(x) release flag is set, the routine goes from step 440 to step 448, where a second calculation flag is set. The second calculation flag is set when a rich period value CRb for the second subgroup 1bb is once calculated, and is reset when a counter value COUNTb, which represents a period in which the second subgroup 1bb is performing the rich operation is made zero. When it is the first time for the routine to go to step 448 after the second NO_(x) release flag is set, the second calculation flag is reset, and thus the routine goes to step 449, where the rich period value CRb is calculated using the map shown in FIG. 37. In the following step 450, the rich period value CRb is memorized as COUNTb. In the following step 451, the second calculation flag is set. Then, the processing cycle is ended.

When the second calculation flag is set, the routine goes from step 448 to step 452, where the counter value COUNTb is decremented by 1. In the following step 453, it is judged whether the counter value COUNTb is zero. If COUNTb is larger than zero, the processing cycle is ended. Namely, if COUNTb>0, the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11b is judged to be still small, and thus the rich operation of the second subgroup 1bb is continued.

If COUNTb=0 in step 453, the routine goes to step 454, where the second NO_(x) release flag is reset. Namely, if COUNTb 0, the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11b is judged to become sufficiently large, and thus the second subgroup 1bs stops the rich operation and starts the lean operation. In the following step 455, the second calculation flag is reset, and then the processing cycle is ended.

Note that Qba(NO_(x)), Qbb(NO_(x)), Fa(NO_(x)), Fb(NO_(x)), CRa, and CRb are stored in the ROM 22 in advance in the form of a map shown in FIGS. 31, 32, 33, 35, 36, and 37, respectively. The other constructions of the exhaust purifying device and the operation thereof are the same as those in the embodiment explained with reference to FIG. 27, and thus the explanations therefor are omitted.

Next, another embodiment for the exhaust gas purifying catalyst 14 will be explained.

The exhaust gas purifying catalyst in the embodiment uses, for example, a honeycomb type substrate made of cordierite, and an alumina layer which act as a carrier for the catalyst is coated on the cell surface of the honeycomb substrate. On this carrier, at least one substance selected from elements belong to the fourth period or the eighth group in the periodic table of elements, such as copper Cu. chrome Cr, vanadium V, titanium Ti, iron Fe, nickel Ni, cobalt Co, platinum Pt, palladium Pd, rhodium Rh and iridium Ir are carried as a catalyst.

If the exhaust gas purifying catalyst formed as in the above mentioned manner is referred as an NH₃ ·NO_(x) purifying catalyst, the NH₃ -NO_(x) purifying catalyst is capable of converting all of the NH₃ component in the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst to N₂ provided that the exhaust gas is in an oxidizing atmosphere and the temperature of the catalyst is within a specific temperature range as determined by the substance being used as the catalyst. Therefore, when the exhaust gas is in an oxidizing atmosphere containing a NH₃ component and flows through the NH₃ ·NO_(x) purifying catalyst in this temperature range, the NH₃ component in the exhaust gas is almost completely resolved, and the exhaust gas flows out from the NH₃ ·NO_(x) purifying catalyst contains no NH₃ component. In the explanation below, this temperature range in which the NH₃ ·NO_(x) purifying catalyst can resolve all the NH₃ component in the exhaust gas is called an optimum temperature range.

When the temperature of the NH₃ ·NO_(x) purifying catalyst is higher than the optimum temperature range, the NH₃ component in the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst is oxidized by the NH₃ ·NO_(x) purifying catalyst and NO_(x) components are produced.

Namely, when the temperature of the NH₃ ·NO_(x) purifying catalyst is higher than the optimum temperature range, the oxidizing reaction of the NH₃ component, i.e., the above-mentioned reactions (7) and (8) become dominant on the NH₃ ·NO_(x) purifying catalyst, and the amount of NO_(x) components, mainly NO and NO₂, in the exhaust gas flowing out from the NH₃ ·NO_(x), purifying catalyst increases.

Further, when the temperature of the NH₃ ·NO_(x) purifying catalyst is lower than the optimum temperature range, the oxidizing reaction of the NH₃ component (7) and (8) becomes lower, and the amount of the NH₃ component in the exhaust gas flowing out from the NH₃ ·NO_(x) purifying catalyst increases.

FIG. 38 schematically illustrates the variation in the characteristics of the NH₃ ·NO_(x) purifying catalyst in accordance with the change in the temperature. FIG. 38 shows the variation in the concentration of the NH₃ and NO_(x) components in the exhaust gas flowing out from the NH₃ ·NO_(x) purifying catalyst in accordance with the temperature of the NH₃ ·NO_(x) purifying catalyst when the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst is in an oxidizing atmosphere and the concentration of NH₃ in the exhaust gas is maintained at a constant level. The vertical axis and the horizontal axis in FIG. 38 represent the concentration of the respective components in the exhaust gas and the temperature of the NH₃ ·NO_(x) purifying catalyst, respectively. The solid line and the dotted line in FIG. 38 represent the concentrations of the NH₃ component and the NO_(x) components in the exhaust gas flowing out from the NH₃ ·NO_(x) purifying catalyst, respectively.

As shown in FIG. 38, provided that the concentration of the NH₃ component in the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst is maintained at a constant level, the concentration of the NH₃ component in the outflow exhaust gas is substantially the same as the concentration of NH₃ in the inflow exhaust gas in the low temperature region (region I in FIG. 38). In this temperature region, the concentration of the NO_(x) components in the outflow exhaust gas is substantially zero. This means that substantially all of the NH₃ component in the exhaust gas passes through the NH₃ ·NO_(x) purifying catalyst without reaction when the temperature is low (region I in FIG. 38).

When the temperature becomes higher than the above low temperature region, the concentration of the NH₃ component in the outflow exhaust gas decreases as the temperature increases, while the concentration of the NO_(x) components is substantially the same (region II in FIG. 38). Namely, in this temperature region, the amount of NH₃ component in the exhaust gas which is converted to N₂ component increases as the temperature increases.

When the temperature further increases, as shown in region III in FIG. 38, the concentration of NH₃ component in the outflow exhaust gas further decreases and the concentration of both the NH₃ and NO_(x) components becomes substantially zero. Namely, in this temperature region (region III in FIG. 38), all of the NH₃ component in the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst is resolved (i.e., converted to N₂ component) by the NH₃ ·NO_(x) purifying catalyst without forming NO_(x) components.

However, when the temperature becomes higher than this region, the concentration of the NO_(x) components in the outflow exhaust gas increases as the temperature increases (region Iv in FIG. 38), and all of the NH₃ component in the exhaust gas is converted to NO_(x) components by the NH₃ ·Nox purifying catalyst in a high temperature region (region V in FIG. 38).

In this specification, the optimum temperature range of the NH₃ ·NO_(x) purifying catalyst is defined as a temperature range in which all of the NH₃ component in the exhaust gas is converted to a N₂ component without forming any NO_(x) component, i.e., such as the temperature range indicated by the temperature region III in FIG. 38.

The optimum temperature range of the NH₃ ·NO_(x) purifying catalyst changes according the substance used as catalytic component, and generally starts at a relatively low temperature compared with, for example, the activating temperature of the TW catalyst. For example, when a substance such as platinum Pt, rhodium Rh, or palladium Pd is used, the optimum temperature range is approximately 100 to 400° C. (preferably 100 to 300° C. and most preferably 100 to 250° C. in case of platinum Pt, and preferably 150 to 400° C. and most preferably 150 to 300° C. in case of rhodium Rh or palladium Pd). When a substance such as copper Cu, chrome Cr, or iron, for example, is used, the optimum temperature range is approximately 150 to 650° C. (preferably 150 to 500° C.). Therefore, if the NH₃ ·NO_(x) purifying catalyst is formed as a tandem compound type catalyst using both types of the catalytic component, i.e., if the catalytic components such as platinum Pt are carried on the downstream part of the substrate and the catalytic components such as chrome Cr are carried on the upstream part of the substrate, the optimum temperature range of the NH₃ ·NO_(x) purifying catalyst can be widened as a whole.

The reason why the NH₃ ·NO_(x) purifying catalyst converts substantially all of the NH₃ component in the exhaust gas to the N₂ component without producing any NO_(x) components only in the specific temperature range is not clear at present. However, it is considered that this phenomena is due to the following reason.

Namely, when the temperature of the NH₃ ·NO_(x) purifying catalyst is in the optimum temperature range, the above mentioned denitrating reactions (9) and (10) occur on the NH₃ ·NO_(x) purifying catalyst, in addition to the above mentioned oxidizing reactions (7) and (8). Due to these denitrating reactions (9) and (10), the NO_(x) components produced by the oxidizing reactions (7) and (8) are immediately converted to the N₂ component. Namely, in the optimum temperature range, a portion of the NH₃ in the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst is converted to NO_(x) by the oxidizing reactions (7) and (8), and this NO_(x) immediately reacts with the remaining NH₃ in the exhaust gas and is converted to N₂ by the denitrating reactions (9) and (10). By these sequential reactions, substantially all of the NH₃ in the exhaust gas is converted to N₂ when the temperature of the catalyst is within the optimum temperature range.

When the temperature of the NH₃ ·NO_(x) purifying catalyst is above the optimum temperature range, the oxidizing reactions (7) and (8) become dominant in the catalyst and the portions of NH₃ which is oxidized by the catalyst increases. Thus, the denitrating reactions (9) and (10) hardly occur in the catalyst due to the shortage of NH₃ component in the exhaust gas, and the NO_(x) produced by the oxidizing reactions (7) and (8) flows out from the NH₃ ·NO_(x) purifying catalyst without being reduced by the denitrating reactions (9) and (10).

On the other hand, when the temperature of NH₃ ·NO_(x) purifying catalyst is below the optimum temperature range, the oxidizing reactions (7) and (8) hardly occur due to the low temperature. This causes the NH₃ in the exhaust gas to pass through the NH₃ ·NO_(x) purifying catalyst without being oxidized by the NO_(x) due to the shortage of the NO_(x) in the exhaust gas.

As explained above, the optimum temperature range of the NH₃ ·NO_(x) purifying catalyst is a temperature range in which the oxidizing reactions of the NH₃ (7) and (8) and the denitrating reactions of the NO_(x) (9) and (10) balance each other in such a manner that the NO_(x) produced by the oxidation of the NH₃ immediately reacts with NH₃ in the exhaust gas without causing any surplus NO_(x) and NH₃. Consequently, the optimum temperature range of the NH₃ ·NO_(x) purifying catalyst is determined by the oxidizing ability of the catalyst and its temperature dependency. Therefore, when the catalyst component having high oxidizing ability, such as platinum Pt, is used, the optimum temperature range becomes lower than that when the catalyst component having relatively low oxidizing ability, such as chrome Cr is used.

As explained above, though the mechanism of the phenomenon is not completely clarified, the NH₃ ·NO_(x) purifying catalyst actually converts all of the NH₃ in the exhaust gas under an oxidizing atmosphere when the temperature is within the optimum temperature range. Further, when the NH₃ ·NO_(x) purifying catalyst is used in the optimum temperature range the following facts were found in connection with the above phenomenon:

(a) When the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst is in an oxidizing atmosphere, i.e., when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean compared to the stoichiometric air-fuel ratio, substantially all of the NH₃ in the exhaust gas is converted to N₂ without producing any NO_(x). This occurs when the exhaust gas is in an oxidizing atmosphere (a lean air-fuel ratio), but regardless of the degree of leanness of the exhaust gas air-fuel ratio of the inflowing exhaust gas.

(b) When the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst contains NO_(x) in addition to NH₃, all of the NO_(x) in the exhaust gas as well as the NH₃ is converted to N₂, and the concentration of the NO_(x) components in the exhaust gas becomes zero. In this case, the ratio of the concentrations of the NO_(x) components and the NH₃ component is not necessarily stoichiometrical for the denitrating reactions (9) and (10) (i.e., 4:3, or 1:1). It is only required that the exhaust gas contains an amount of NH₃ more than the amount required for reducing the NO_(x) (NO₂ and NO) in the exhaust gas. As explained above, since the surplus NH₃ in the exhaust gas is all converted to N₂ when the exhaust gas is in an oxidizing atmosphere, no surplus NH₃ is contained in the exhaust gas flowing out from the NH₃ ·NO_(x) purifying catalyst even in this case.

(c) When the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst contains HC and CO components, all of the HC and CO components are oxidized by the NH₃ ·NO_(x) purifying catalyst, provided that the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean compared to the stoichiometric air-fuel ratio, and no HC and CO components are contained in the exhaust gas flowing out from the NH₃ ·NO_(x) purifying catalyst.

However, when the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst contains both the NH₃ and NO_(x), it was found that the temperature region Iv in FIG. 38, i.e., the temperature region in which the concentration of NO_(x) components in the outflow exhaust gas increases as the temperature of the catalyst increases, moves to the lower temperature side compared to that when the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst contains only the NH₃ components. This is because, when the exhaust gas contains NO_(x) in addition to NH₃, the NO_(x) in the inflow exhaust gas in addition to the NO_(x) produced by the oxidizing reaction of NH₃ must be reduced by the NH₃ in the exhaust gas. Consequently, the shortage of NH₃ is apt to occur in the relatively low temperature region. Therefore, when the exhaust gas contains both the NH₃ and the NO_(x), the optimum temperature range of the NH₃ ·NO_(x) purifying catalyst becomes narrower.

In relation to above (b), a conventional denitrating catalyst, such as a vanadia-titania V₂ O₅ --TiO₂ type catalyst also has a capability for resolving NH₃ and NO_(x) in the exhaust gas with a certain conditions. However, in case of the conventional denitrating catalyst, the amounts of NH₃ and NO_(x) components must be strictly stoichiometrical in order to react NH₃ with NO_(x) without causing any surplus NH₃ and NO_(x). Namely, when both the NO₂ and NO are contained in the exhaust gas, the amount (moles) of the NH₃ in the exhaust gas must be strictly equal to the total of the moles of NO₂ in the exhaust gas multiplied by 3/4 and the moles of NO in the exhaust gas to react NH₃ and NO_(x) without causing any surplus NH₃ and NO_(x). However, in case of the NH₃ ·NO_(x) purifying catalyst in the embodiment, if the amount of the NH₃ is more than stoichiometrical compared to the amount of NO_(x), and if the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, all of the NH₃ and NO_(x) are converted to N₂ without causing any surplus NH₃ and NO_(x). This is an important difference between the NH₃ ·NO_(x) purifying catalyst in the present invention and the conventional denitrating catalyst.

As explained in FIG. 38, though the NH₃ ·NO_(x) purifying catalyst converts all of the NH₃ in the exhaust gas in the optimum temperature range, some NH₃ passes through when the temperature is below the optimum temperature range. In order to prevent this outflow of NH₃ in the low temperature region, an acidic inorganic substance may be used. It is known in the art that an acidic inorganic substance (which includes Broensted acids such as zeolite, silica SiO₂, silica-alumina SiO₂ --Al₂ O₃, and titania TiO₂ as well as Lewis acids including oxides of transition metals such as copper Cu, cobalt Co, nickel Ni and iron Fe) absorb NH₃ when the temperature is low. Therefore, one or more of these acidic inorganic substances may be carried on the substrate of the NH₃ ·NO_(x) purifying catalyst, or the substrate itself may be formed by a porous material made of such acidic inorganic substances to prevent the outflow of NH₃ in the low temperature region. In this case, the NH₃ component which is not converted to an N₂ component in the temperature region below the optimum temperature range is absorbed by the acidic inorganic substances in the NH₃ ·NO_(x) purifying catalyst, and the amount of the outflow of the NH₃ from the NH₃ ·NO_(x) purifying catalyst in the low temperature region can be reduced. The NH₃ absorbed by the acidic inorganic substances are released when the temperature of the NH₃ ·NO_(x) purifying catalyst becomes high, or when the concentration of NH₃ component in the exhaust gas becomes low. Therefore, the NH₃ absorbed by the acidic inorganic substance is converted to N₂ by the NH₃ ·NO_(x) purifying catalyst when it is desorbed from the acidic inorganic substance. When the temperature of the exhaust gas flowing into the NH₃ ·NO_(x) purifying catalyst changes in a wide range, therefore, it is suitable to use these acidic inorganic substances to prevent the outflow of NH₃ in low temperature region.

Further, as long as such desorption occurs, the adsorbed NH₃ amount in the acidic inorganic substance does not increase. As a result, the NH₃ ·NO_(x) purifying catalyst is prevented from being saturated with NH₃, that is, NH₃ is prevented from flowing out from the NH₃ ·NO_(x) purifying catalyst without being purified. This means that there is no need to arrange the NH₃ purifying catalyst downstream of the NH₃ ·NO_(x) purifying catalyst, and this simplifies the structure of the exhaust gas purifying catalyst.

Next, another embodiment of the exhaust gas purifying catalyst 14 will be explained with reference to FIG. 39. In FIG. 39, constituent elements the same as those in the above-mentioned embodiments are given the same reference numerals.

Referring to FIG. 39, the exhaust gas purifying catalyst 14 is provided with three catalysts arranged in series. The catalysts are, from upstream side, in turn, the Cu zeolite catalyst 141, the Pt--Cu zeolite catalyst 142, and the precious metal catalyst 143. Note that the catalysts 141, 142, and 143 are housed in corresponding catalytic converter 151, 152, and 153. Further, the inlet of the converter 151 is connected to the outlet of the interconnecting duct 13.

According to the inventors of the present invention, it has been found that the upper limit temperature of the optimum temperature range under the oxidizing atmosphere of the precious metal catalyst 143 is highest in the catalysts 141, 142, and 143, that of the Pt--Cu zeolite catalyst 142 is next to that of the precious metal catalyst 143, and that of the Cu zeolite catalyst 141 is lowest. On the other hand, an exhaust gas temperature at an outlet of an catalyst may become higher than that at an inlet, due to the reaction occurring on the catalyst. Thus, the Cu zeolite catalyst 141, the Pt--Cu zeolite catalyst 142, and the precious metal catalyst 143 are arranged in turn, from the upstream side, in this embodiment. This prevents unusual deterioration of the catalysts, while ensuring good purification of the exhaust gas.

Next, another embodiment of the exhaust gas purifying catalyst 14 will be explained with reference to FIG. 40. In FIG. 40, constituent elements the same as those in the above-mentioned embodiments are given the same reference numerals. Further, an electronic control unit same as shown in FIG. 1 is also provided in this embodiment, but it is depicted simply by a box, in FIG. 40.

Referring to FIG. 40, the exhaust gas purifying catalyst 14 is provided with the precious metal catalyst 143 housed in the catalytic converter 156, and the Cu zeolite catalyst 141 and the Pt--Cu zeolite catalyst 142 housed in the common catalytic converter 157, which is arranged downstream of the converter 157. The Cu zeolite catalyst 141 and the Pt--Cu zeolite catalyst 142 are carried on a common substrate, and are arranged in series, in turn, with respect to the exhaust gas flow. Formed in the catalytic converter 156 are first and second passages 161 and 162, separated by a separating wall 160. The inlets of the first and second passages 161 and 162 are connected to the outlet of the interconnecting duct 13, and the outlets are connected to the inlet of the catalytic converter 157 via an exhaust gas control valve 164. As shown in FIG. 40, the precious metal catalyst 143 is arranged in the first passage 161.

The exhaust gas control valve 164 is arranged in the catalytic converter 156, and is driven by an actuator 163 of a solenoid or vacuum type. When the exhaust gas control valve 164 is positioned at a position shown by the solid line in FIG. 40, the first passage 161 is opened and the second passage 162 is closed, and thereby the interconnecting passage 13 communicates with the catalytic converter 157 via the first passage 161. When the exhaust gas control valve 164 is positioned to a position shown by the broken line in FIG. 40, the first passage 161 is closed and the second passage 162 is opened, and thereby the interconnecting passage 13 communicates with the catalytic converter 157 via the second passage 162. Note that the actuator 163 is connected, via a drive circuit, to the output port of the ECU 20, and is controlled in accordance with the output signals from the ECU 20.

Next, the control of the exhaust gas control valve will be explained with reference to FIG. 41. The routine shown in FIG. 41 is executed by interruption every predetermined time.

Referring to FIG. 41, first, in step 170, it is judged whether the low load operation, including the idling operation, is in process at this time. If the low load operation is in process, the routine goes to step 171, where the exhaust gas control valve 164 is positioned to the position shown by the solid line in FIG. 40, and thus the interconnecting duct 13 is connected to the first passage 161. Namely, the exhaust gas from the duct 13 contacts, in turn, the precious metal catalyst 143, the Cu zeolite catalyst 141, and the Pt--Cu zeolite catalyst 142.

In the low load engine operation, the temperature of the exhaust gas flowing into the exhaust gas purifying catalyst 14 is low. Thus, if the low load operation is continued for a long period, the temperatures of the Cu zeolite catalyst 141 and the Pt--Cu zeolite catalyst 142 may become lower and the purification ability of the catalysts 141 and 142 may be lowered. Therefore, in this embodiment, the exhaust gas, first, makes contact with the precious metal, in the low load operation, to prevent the temperature of the exhaust gas from dropping as much as possible, to thereby ensure the purification ability of the catalysts 141, 142, and 143. As a result, the exhaust gas is purified sufficiently by the catalysts 141, 142, and 143.

Contrarily, if the low load operation is not in process, that is, if the middle or high load operation is in process, in step 170, the routine goes to step 172, where the exhaust gas control valve 164 is positioned to the position shown by the broken line in FIG. 40, and thus the interconnecting duct 13 is connected to the second passage 162. Namely, the exhaust gas from the duct 13 bypasses the precious metal catalyst 143, and then contacts with, in turn, the Cu zeolite catalyst 141 and the Pt--Cu zeolite catalyst 142.

When the exhaust gas of which the temperature is high flows into the precious metal catalyst 143, the oxidizing reactions (9) and (10) mentioned above become dominant thereon, and the large amount of NO_(x) may flow out therefrom. Such a large amount of NO_(x) may not be purified on the following Cu zeolite catalyst 141 and Pt--Cu zeolite catalyst 142. Therefore, in this embodiment, during the middle or high load operation where the exhaust gas temperature is relatively high, the exhaust gas bypasses the precious metal catalyst 143, and contacts the Cu zeolite catalyst 141 and Pt--Cu zeolite catalyst 142, to thereby prevent NO_(x) from flowing out from the exhaust gas purifying catalyst 14.

While, in this embodiment, the exhaust gas control valve 164 is controlled in accordance with the engine load, the valve 164 may be controlled in accordance with the temperature of the inflowing exhaust gas or each catalyst, or with the engine operating condition such as the engine speed.

FIG. 42 illustrates another embodiment of the exhaust gas purifying device. In FIG. 42, constituent elements the same as those in the above-mentioned embodiments are given the same reference numerals.

Referring to FIG. 42, the first cylinder group 1a is connected, via the exhaust duct 7, to the catalytic converter 9 housing the TW catalyst 8a therein, and the converter 9 is selectively connected, via an NH₃ switching valve 200a, to either a first NH₃ introducing duct 201a or a second NH₃ introducing duct 201b. Also, the second cylinder group 1b is connected, via the exhaust manifold 10, to an exhaust duct 210, and the duct 210 is selectively connected, via a NO_(x) switching valve 200b, to either a first NO_(x) introducing duct 202a or a second NO_(x) introducing duct 202b. The first NH₃ introducing duct 201a and the second NO_(x) introducing duct 202b are connected to a common catalytic converter 203 housing an NH₃ -AO catalyst 204 therein. The second NH₃ introducing duct 201b and the first NO_(x) introducing duct 202a are connected to a common catalytic converter 205 housing a NO_(x) -OR catalyst 206 therein. The converters 203 and 205 are connected, via an interconnecting duct 207, to the common catalytic converter 17 housing the NH₃ purifying catalyst 16.

The NH₃ switching valve 200a and the NO_(x) switching valve 200b are controlled by a common actuator 208 of solenoid or vacuum type. The actuator 208 drives the switching valves 200a and 200b simultaneously, to connect the TW catalyst 8a to either the first or the second NH₃ introducing duct 201a, 201b, selectively, and to connect the exhaust duct 210 to either the first or the second NO_(x) introducing duct 202a, 202b, selectively. Note that the actuator 208 is connected, via the drive circuit 32, to the output port 26 of the ECU 20, and is controlled in accordance with the output signals from the ECU 20.

In this embodiment, the NH₃ -AO catalyst 204 is formed as the NH₃ -AO catalyst 14a in the above-mentioned embodiments, and the NO_(x) -OR catalyst 206 is formed as the NO_(x) -OR catalyst 11a in the above-mentioned embodiments. Alternatively, the NH₃ -AO catalyst 204 may be formed as the NH₃ ·NO_(x) purifying catalyst including the acidic inorganic substance, as explained with reference to FIG. 38.

In this embodiment, the first group 1a continuously performs the rich operation to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a rich continuously. Therefore, the exhaust gas including NH₃, of which the exhaust gas air-fuel ratio is rich flows into the first or the second NH₃ introducing duct 201a, 201b. Also, the second group 1b continuously performs the lean operation to make the exhaust gas air-fuel ratio of the exhaust gas flowing out from the exhaust duct 210 lean continuously. Therefore, the exhaust gas including NO_(x), of which the exhaust gas air-fuel ratio is lean flows into the first or the second NO_(x) introducing duct 202a, 202b. Note that the target air-fuel ratio (A/F)T for the first and the second groups 1a and 1b are set as in a manner to set the rich and the lean air-fuel ratios (A/F)R and (A/F)L, mentioned above.

In this embodiment, the exhaust gas of the engine 1 is purified by performing an adsorbing and occluding process and a desorbing, releasing, and purifying process, alternately and repeatedly. First, the adsorbing and occluding process will be explained with reference to FIGS. 43A and 43B.

In the adsorbing and occluding process, the NH₃ switching valve 200a opens the first NH₃ introducing duct 201a and closes the second NH₃ introducing duct 201b. At the same time, the NO_(x) switching valve 200b opens the first NO_(x) introducing duct 202a and closes the second NO_(x) introducing duct 202b. As a result, the TW catalyst 8a is connected, via the first NH₃ duct 201a, to the NH₃ -AO catalyst 204 and the duct 210 is connected, via the first NO_(x) duct 202a, to the NO_(x) -OR catalyst 206. Such a connecting condition of the TW catalyst 8a and the duct 210 is referred to as a first connecting condition.

Namely, in the adsorbing and occluding process, the exhaust gas including NH₃, of which the exhaust gas air-fuel ratio is rich flows into the NH₃ -AO catalyst 204. Substantially all of NH₃ in the exhaust gas is adsorbed in the NH₃ -AO catalyst 204. In this condition, even if NH₃ flows out from the NH₃ -AO catalyst 204 without being adsorbed, the NH₃ is purified on the following NH₃ purifying catalyst 16. On the other hand, the exhaust gas including NO_(x), of which the exhaust gas air-fuel ratio is lean flows into the NO_(x) -OR catalyst 206. Substantially all of NO_(x) in the exhaust gas is occluded in the NO_(x) -OR catalyst 206. Accordingly, NH₃ and NO_(x) are prevented from flowing downstream of the NH₃ purifying catalyst 16, in the adsorbing and occluding process.

Contrarily, in the desorbing, releasing, and purifying process, as shown in FIGS. 44A and 44B, the NH₃ switching valve 200a closes the first NH₃ introducing duct 201a and opens the second NH₃ introducing duct 201b. At the same time, the NO_(x) switching valve 200b closes the first NO_(x) introducing duct 202a and opens the second NO_(x) introducing duct 202b. As a result, the TW catalyst 8a is connected, via the second NH₃ duct 201b, to the NO_(x) -OR catalyst 206 and the duct 210 is connected, via the second NO_(x) duct 202b, to the NH₃ -AO catalyst 204. Such a connecting condition of the TW catalyst 8a and the duct 210 is referred to as a second connecting condition.

Namely, in the desorbing, releasing, and purifying process, the exhaust gas including NO_(x) without including NH₃, of which the exhaust gas air-fuel ratio is lean, flows into the NH₃ -AO catalyst 204. As a result, the adsorbed NH₃ is desorbed from the NH₃ -AO catalyst 204, and the desorbed NH₃ reduces or purifies the inflowing NO_(x). In this condition, even if the desorbed NH₃ amount is excessive to the inflowing NO_(x) amount, the excess NH₃ is purified on the following NH₃ purifying catalyst 16. On the other hand, the exhaust gas including NH₃, of which the exhaust gas air-fuel ratio is rich flows into the NO_(x) -OR catalyst 206. As a result, the occluded NO_(x) is released from the NO_(x) -OR catalyst 206, and the released NO_(x) is reduced or purified by the inflowing NH₃. In this condition, even if the inflowing NH₃ amount is excessive to the released NO_(x) amount, the excess NH₃ is purified on the following NH₃ purifying catalyst 16. Accordingly, NH₃ and NO_(x) are prevented from flowing downstream of the NH₃ purifying catalyst 16, regardless whether in the adsorbing and occluding process, or in the desorbing, releasing, and purifying process.

In this way, performing the adsorbing and occluding process, and the desorbing, releasing, and purifying process alternately and repeatedly, provide good purification of the exhaust gas. Further, the second group 1b continuously performs the lean operation, in this embodiment, and this makes the fuel consumption rate lower.

When the NH₃ switching valve 200a and the NO_(x) switching valve 200b are controlled and the connecting conditions of the TW catalyst 8a and the duct 210 are made the first connecting condition, the adsorbing and occluding process is performed, and when they are made the second connecting condition, the desorbing, releasing, and purifying process is performed. A switching control of the processes, that is, a switching control of the connecting conditions may be executed at any timing, as long as the saturation of the NH₃ -AO catalyst 204 and the NO_(x) -OR catalyst 206 are prevented. However, a frequent switching of the connecting condition is undesirable. Thus, in this embodiment, the NH₃ amount S1(NH₃) adsorbed in the NH₃ -AO catalyst 204, or the NO_(x) amount S1(NO_(x)) occluded in the NO_(x) -OR catalyst 206 is found, and the switching control of the connecting condition is executed in accordance with S1(NH₃) or S1(NO_(x)).

Namely, in the adsorbing and occluding process, when the at least one of the adsorbing NH₃ amount S1(NH₃) and the occluding NO_(x) amount S1(NO_(x)) becomes larger than the corresponding upper threshold amount UT1(NH₃), UT1(NO_(x)), the desorbing, releasing, and purifying process is started. Also, in the desorbing, releasing, and purifying process, when the at least one of the adsorbing NH₃ amount S1(NH₃) and the occluding NO_(x) amount S1(NO_(x)) becomes smaller than the corresponding lower threshold amount LT1(NH₃), LT1(NO_(x)), the adsorbing and occluding process is started. Accordingly, the exhaust gas is sufficiently purified without frequent switching in the connecting conditions, while preventing the saturation of the NH₃ -AO catalyst 204 and the NO_(x) -OR catalyst 206.

FIG. 45 illustrates a routine for executing the switching control of the connecting conditions or the processes, according to the embodiment. The routine is executed by interruption every predetermined time.

Referring to FIG. 45, first, in step 220, the adsorbed NH₃ amount S1(NH₃) is calculated (explained below). In the following step 221, the occluded NO_(x) amount S1(NO_(x)) is calculated (explained below). In the following step 222, it is judged whether the current connecting condition is the second connecting condition, that is, the desorbing, releasing, and purifying process is in process. If the current connecting condition is the first connecting condition, that is, the adsorbing and occluding process is in process, the routine goes to step 223, where it is judged whether S1(NH₃) is larger than the predetermined, upper threshold amount UT1(NH₃). If S1(NH₃)>UT1(NH₃), the routine jumps to step 225. If S1(NH₃)≦UT1(NH₃), the routine goes to step 224.

In the step 224, it is judged whether S1(NO_(x)) is larger than the predetermined, upper threshold amount UT1(NO_(x)). If S1(NO_(x))≦UT1(NO_(x)), the processing cycle is ended. Namely, the first connecting condition or the adsorbing and occluding process is continued. If S1(NO_(x))>UT1(NO<), the routine goes to step 225. Thus, the routine goes to step 225, when S1(NH₃)>UT1(NH₃) or when S1(NO_(x))>UT1(NO_(x)). In the step 225, the connecting condition is changed to the second connecting condition, and the desorbing, releasing, and purifying process is started. Then, the processing cycle is ended.

If the current connecting condition is the second connecting condition, that is, the desorbing, releasing, and purifying process is in process, in step 222, the routine goes to step 226, where it is judged whether S1(NH₃) is smaller than the predetermined, lower threshold amount LT1(NH₃). If S1(NH₃)<LT1(NH₃), the routine jumps to step 228. If S1(NH₃)≧LT1(NH₃), the routine goes to step 227. In the step 227, it is judged whether S1(NO_(x)) is smaller than the predetermined, lower threshold amount LT1(NO_(x)). If S1(NO_(x))≧LT1(NO_(x)), the processing cycle is ended. Namely, the second connecting condition or the desorbing, releasing, and purifying process is continued. If S1(NO_(x))<LT1(NO_(x)), the routine goes to step 228. Thus, the routine goes to step 228, when S1(NH₃)<LT1(NH₃) or when SL(NO_(x))<LT1(NO_(x)). In step 228, the connecting condition is changed to the first connecting condition, and the adsorbing and occluding process is started. Then, the processing cycle is ended.

In the step 220 in the routine shown in FIG. 45, when the connecting condition is the first connecting condition and the adsorbing and occluding process is in process, the adsorbed NH₃ amount S1(NH₃) is calculated by integrating the product of the NO_(x) amount Qa(NO_(x)) exhausted from the first group 1a per unit time and the NH₃ synthesizing efficiency ETA, over time. The exhausted NO_(x) amount Qa(NO_(x)) and the efficiency ETA are obtained using the maps shown in FIGS. 13B and 14, respectively. When the connecting condition is the second connecting condition and the desorbing, releasing, and purifying process is in process, the adsorbed NH₃ amount S1(NH₃) is calculated by integrating the NH₃ amount D(NH₃) desorbed from the NH₃ -AO catalyst 204 per unit time, with time. The desorbed NH₃ amount D(NH₃) is obtained using the map shown in FIG. 16B.

In the step 221 in the routine shown in FIG. 45, when the connecting condition is the first connecting condition and the adsorbing and occluding process is in process, the occluded NO_(x) amount S1(NO_(x)) is calculated by integrating the NO_(x) amount Qb(NO_(x)) exhausted from the second group 1b per unit time, with time. The exhausted NO_(x) amount Qb(NO_(x)) is obtained using the map shown in FIG. 6B. When the connecting condition is the second connecting condition and the desorbing, releasing, and purifying process is in process, the occluded NO_(x) amount S1(NO_(x)) is calculated by integrating the NO_(x) amount D(NO_(x)) released from the NO_(x) -OR catalyst 206 per unit time, over time. The released NO_(x) amount D(NO_(x)) is obtained using the map shown in FIG. 8B.

FIG. 46 illustrates another embodiment of the exhaust gas purifying device. In FIG. 46, constituent elements the same as those in the above-mentioned embodiments are given the same reference numerals.

Referring to FIG. 46, this embodiment is different from the above embodiment shown in FIG. 42 in the point that the occlusive material is provided in the exhaust passage between the exhaust manifold 10 and the NO_(x) switching valve 200b, and that the secondary air supplying device 18 is provided in the exhaust passage between the occlusive material 11 and the NO_(x) switching valve 200b. Thus, the exhaust gas from the second group 1b, first, contacts the occlusive material 11, and then contacts the NH₃ -AO catalyst 204 or the NO_(x) -OR catalyst 206.

As in the embodiment explained above with reference to FIG. 1, the occlusive material 11 is for preventing the large amount of NO_(x) from flowing into the NH₃ -AO catalyst 204 or the NO_(x) -OR catalyst 206. Note that, as the occlusive material 11, the NO_(x) -OR catalyst 11a is used.

In this embodiment again, the adsorbing and occluding process, and the desorbing, releasing, and purifying process are performed alternately and repeatedly. In the desorbing, releasing, and purifying process, the exhaust gas exhausted from the second group 1b and including NO_(x) flows, via the second NO_(x) introducing duct 202b, into the NH₃ -AO catalyst 204. In this case, if a large amount of NO_(x) flows into the NH₃ -AO catalyst 204, the NO_(x) may be excessive to the NH₃ desorbed from the NH₃ -AO catalyst 204, and the excess NO_(x) may flow out from the NH₃ -AO catalyst 204 without being purified. Also, in the adsorbing and occluding process, the exhaust gas exhausted from the second group 1b flows, via the first NO_(x) introducing duct 202a, into the NO_(x) -OR catalyst 206. In this case, if a large amount of NO_(x) flows into the NO_(x) -OR catalyst 206, NO_(x) may flow out from the NH₃ -AO catalyst 204 without being purified, even though the NO_(x) -OR catalyst 206 has a NO_(x) occluding ability. Thus, the occlusive material 11 is arranged between the second group 1b and the NH₃ -AO catalyst 204 and the NO_(x) -OR catalyst 206, to thereby prevent a large amount of NO_(x) from flowing into the NH₃ -AO catalyst 204 and the NO_(x) -OR catalyst 206. This prevents NO_(x) from flowing out from the catalysts 204 and 206.

The switching control of the connecting conditions of the processes are executed in accordance with the adsorbed NH₃ amount in the NH₃ -AO catalyst 204 and the occluded NO_(x) amount in the NO_(x) -OR catalyst 206, as in the above embodiment explained with reference to FIGS. 42 to 45.

As mentioned above, if the second group 1b continuously performs the lean operation, the occluded NO_(x) amount in the NO_(x) -OR catalyst 11a becomes larger, and the NO_(x) occluding capacity becomes smaller. Therefore, the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11 is obtained, and when the occluded NO_(x) amount (NO_(x)) becomes larger than the upper threshold UT(NO_(x)), the second group 1b performs the rich operation temporarily to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst 11a rich, to thereby release the occluded NO_(x) from the NO_(x) -OR catalyst 11a. This ensures the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a. When the occluded NO_(x) amount S(NO_(x)) becomes smaller than the lower threshold LT(NO_(x)), the second group 1b resumes the lean operation.

When the second group 1b performs the rich operation, the exhaust gas including NO_(x), of which the exhaust gas air-fuel ratio is rich, flows out from the NO_(x) -OR catalyst 11a. On the other hand, the temporary rich operation in the second group 1b to release the occluded NO_(x) from the NO_(x) -OR catalyst 11a is performed regardless the connecting conditions. Thus, the exhaust gas including NO_(x), of which the exhaust gas air-fuel ratio is rich, flows into the NO_(x) -OR catalyst 206 in the adsorbing and occluding process, and into the NH₃ -AO catalyst 204 in the desorbing, releasing, and purifying process. However, if the exhaust gas of which the exhaust gas air-fuel ratio is rich flows into the NO_(x) -OR catalyst 206 in the adsorbing and occluding process, the occluded NO_(x) is released from the NO_(x) -OR catalyst 206, which is not desirable. If the exhaust gas of which the exhaust gas air-fuel ratio is rich flows into the NH₃ -AO catalyst 204, NO_(x) and NH₃ will not be purified on the NH₃ -AO catalyst 204 sufficiently, even if NH₃ is desorbed from the catalyst 204.

Thus, in this embodiment, the secondary air supply device 18 supplies the secondary air when the second group 1b performs the rich operation, and thereby the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ -AO catalyst 204 or the NO_(x) -OR catalyst 206 is kept lean. Further, the secondary air also keeps the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ purifying catalyst 16 lean. Accordingly, good purification of the exhaust gas on the catalysts is ensured.

FIG. 47 illustrates a routine for executing the operation change control of the second group 1b, according to the embodiment. The routine is executed by interruption every predetermined time.

Referring to FIG. 47, first, in step 460, the occluded NO_(x) amount S(NO_(x)) in the NO_(x) -OR catalyst 11a is calculated. The method for calculating S(NO_(x)) is the same as that in the embodiments explained above, and thus the explanation thereof is omitted. In the following step 461, it is judged whether a NO_(x) flag is set. The NO_(x) flag is set when the second group 1b has to perform the rich operation to release the occluded NO_(x) from the NO_(x) -OR catalyst 11a, and is reset when the second group 1b has to perform the lean operation. If the NO_(x) release flag is reset, that is, the second group 1b has to perform the lean operation, the routine goes to step 462, where it is judged whether the occluded NO_(x) amount S(NO_(x)) is larger than the upper threshold UT(NO_(x)). If S(NO_(x))≦UT(NO_(x)), the processing cycle is ended. Namely, if S(NO_(x))≦UT(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst h1a is judged to be still large, and thus the lean operation of the second group 1b is continued.

If S(NO_(x))>UT(NO_(x)), the routine goes to step 463, where the NO_(x) release flag is set. Namely, if S(NO_(x))>UT(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to become small. Thus, the second group 1b stops the lean operation and starts the rich operation. In the following step 464, the supply of the secondary air by the secondary air supply device 18 is started. Then, the processing cycle is ended.

If the NO_(x) release flag is set in step 461, the routine goes to step 465, where it is judged whether the occluded NO_(x) amount S(NO_(x)) is smaller than the lower threshold LT(NO_(x)). If S(NO_(x))≧LT(NO_(x)), the processing cycle is ended. Namely, if S(NO_(x))≧LT(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to be still small, and thus the rich operation of the second group 1b is continued.

If S(NO_(x))<LT(NO_(x)), the routine goes to step 466, where the NO_(x) release flag is reset. Namely, if S(NO_(x))<LT(NO_(x)), the NO_(x) occluding capacity of the NO_(x) -OR catalyst 11a is judged to become sufficient. Thus, the second group 1b stops the rich operation and starts the lean operation. In the following step 467, the supply of the secondary air by the secondary air supply device 18 is stopped. Then, the processing cycle is ended.

Note that the other constructions of the exhaust purifying device and the operation thereof are the same as those in the embodiment explained with reference to in FIGS. 42 to 45, and thus the explanations therefor are omitted.

FIG. 48 illustrates another embodiment of the exhaust gas purifying device. In FIG. 48, constituent elements the same as those in the above-mentioned embodiments are given the same reference numerals.

Referring to FIG. 48, the first group 1a is connected to a catalytic converter 301 housing a first NH₃ synthesizing catalyst 300 therein, and the converter 301 is connected to a catalytic converter 303 housing an NH₃ -AO catalyst 302 therein. The second group 1b is connected to a catalytic converter 305 housing a second NH₃ synthesizing catalyst 304 therein, and the converter 305 is connected to a catalytic converter 307 housing a NO_(x) -OR catalyst 306 therein. The converters 303, 307 are connected, via an interconnecting duct 308, to the catalytic converter 17 housing the NH₃ purifying catalyst 16. Further, as shown in FIG. 48, the air-fuel ratio sensor 31 for controlling the engine air-fuel ratio of the first group 1a is arranged in the interconnecting duct 308 just downstream of the NH₃ -AO catalyst 302, and the air-fuel ratio sensor 32 for controlling the engine air-fuel ratio of the second group 1b is arranged in the interconnecting duct 308 just downstream of the NO_(x) -OR catalyst 306.

The first and the second NH₃ synthesizing catalysts 300a and 304a are provided with the TW catalysts, respectively. The TW catalysts 300a and 304a, the NH₃ -AO catalyst 302, and the NO_(x) -OR catalyst 306 are formed as in the embodiments mentioned above, and the explanation thereof are omitted.

Next, the exhaust gas purifying method in this embodiment will be explained, with reference to FIGS. 49 and 50.

In this embodiment, the adsorbing and occluding process and the desorbing, releasing, and purifying process are performed alternately and repeatedly. First, the adsorbing and occluding process will be explained with reference to FIG. 49.

In the adsorbing and occluding process, the first group 1a performs the rich operation to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 300a rich. The NO_(x) exhausted from the first group 1a flows into the TW catalyst 300a and is converted to NH₃. The NH₃ then flows into the NH₃ -AO catalyst 302 and is adsorbed therein.

The second group 1b performs the lean operation to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 304a and the NO_(x) -OR catalyst 306 lean. The NO_(x) exhausted from the second group 1b passes through the TW catalyst 304a without being converted to NH₃, and then flows into the NO_(x) -OR catalyst 306, and is occluded therein. Such a condition of the exhaust gas air-fuel ratio, in which the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 300a is made rich and that of the exhaust gas flowing into the TW catalyst 304a is made lean, is referred as a first exhaust gas air-fuel ratio condition.

Contrarily, in the desorbing, releasing, and purifying process, as shown in FIG. 50, the first group 1a performs the lean operation to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 300a lean. The NO_(x) exhausted from the first group 1a passes through the TW catalyst 300a without being converted to NH₃, and then flows into the NH₃ -AO catalyst 302. As a result, the absorbed NH₃ is desorbed therefrom, and is reduced the inflowing NO_(x). Thus, NO_(x) and NH₃ are purified.

The second group 1b performs the rich operation to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 304a and the NO_(x) -OR catalyst 306 rich. The NO_(x) exhausted from the second group 1b flows into the TW catalyst 304a and is converted to NH₃. The NH₃ then flows into the NH₃ -AO catalyst 306. The exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and the occluded NO_(x) is released from the NO_(x) -OR catalyst 306, the released NO_(x) is reduced by the inflowing NH₃. Such a condition of the exhaust gas air-fuel ratio, in which the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 300a is made lean and that of the exhaust gas flowing into the TW catalyst 304a is made rich, is referred as a second exhaust gas air-fuel ratio condition.

The NH₃ flowing out from the NH₃ -AO catalyst 302 without being adsorbed or purified, or flowing out from the NO_(x) -OR catalyst 306 without being purified, is purified on the following NH₃ purifying catalyst 16. Accordingly, NH₃ is prevented from flowing out from the NH₃ purifying catalyst 16, regardless the process being in process.

Namely, the exhaust gas can be purified by performing the adsorbing and occluding process, and the desorbing, releasing, and purifying process, alternately and repeatedly.

A switching control of the processes, that is, a switching control of the exhaust gas air-fuel ratio conditions may be executed in accordance with the NH₃ amount S2(NH₃) adsorbed in the NH₃ -AO catalyst 302, and/or the NO_(x) amount S2(NO_(x)) occluded in the NO_(x) -OR catalyst 306, as in the above embodiment.

FIG. 51 illustrates a routine for executing the switching control of the exhaust gas air-fuel ratio conditions or the processes, according to the embodiment. The routine is executed by interruption every predetermined time.

Referring to FIG. 51, first, in step 320, the adsorbed NH₃ amount S2(NH₃) is calculated, as in the same manner for calculating S1(NH₃). In the following step 321, the occluded NO_(x) amount S2(NO_(x)) is calculated, as in the same manner for calculating S1(NO_(x)). In the following step 322, it is judged whether the current exhaust gas air-fuel ratio condition is the second exhaust gas air-fuel ratio condition, that is, the desorbing, releasing, and purifying process is in process. If the current exhaust gas air-fuel ratio condition is the first exhaust gas air-fuel ratio condition, that is, the adsorbing and occluding process is in process, the routine goes to step 323, where it is judged whether S2(NH₃) is larger than the predetermined upper threshold amount UT2(NH₃). If S2(NH₃)>UT2(NH₃), the routine jumps to step 325. If S2(NH₃)≦UT2(NH₃), the routine goes to step 324.

In step 324, it is judged whether S2(NO_(x)) is larger than the predetermined, upper threshold amount UT2(NO_(x)). If S2(NO_(x))≦UT2(NO_(x)), the processing cycle is ended. Namely, the first exhaust gas air-fuel ratio condition or the adsorbing and occluding process is continued. If S2(NO_(x))>UT2(NO_(x)), the routine goes to step 325. Thus, the routine goes to step 325, when S2(NH₃)>UT2(NH₃) or when S2(NO_(x))>UT2(NO_(x)). In the step 325, the exhaust gas air-fuel ratio condition is changed to the second exhaust gas air-fuel ratio condition, and the desorbing, releasing, and purifying process is started. Then, the processing cycle is ended.

If the current exhaust gas air-fuel ratio condition is the second exhaust gas air-fuel ratio condition, that is, the desorbing, releasing, and purifying process is in process, in step 322, the routine goes to step 326, where it is judged whether S2(NH₃) is smaller than the predetermined, lower threshold amount LT2(NH₃). If S2(NH₃)<LT2(NH₃), the routine jumps to step 328. If S2(NH₃)≧LT2(NH₃), the routine goes to step 327. In the step 327, it is judged whether S2(NO_(x)) is smaller than the predetermined, lower threshold amount LT2(NO_(x)). If S2(NO_(x))≧LT2(NO_(x)), the processing cycle is ended. Namely, the second exhaust gas air-fuel ratio condition or the desorbing, releasing, and purifying process is continued. If S2(NO_(x))<LT2(NO_(x)), the routine goes to step 328. Thus, the routine goes to step 328, when S2(NH₃)<LT2(NH₃) or when S2(NO_(x))<LT2(NO_(x)). In the step 328, the exhaust gas air-fuel ratio condition is changed to the first exhaust gas air-fuel ratio condition, and the adsorbing and occluding process is started. Then, the processing cycle is ended.

FIG. 52 illustrates another embodiment of the exhaust gas purifying device shown in FIG. 48. In this embodiment, the first and the second NH₃ synthesizing catalysts 300 and 304 are formed by the NO_(x) -OR catalysts 300b and 304b, respectively. This is a difference between the embodiment shown in FIG. 48 and this embodiment.

As mentioned above, the NO_(x) -OR catalyst synthesizes NH₃ from the inflowing NO_(x), when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich. Thus, the NO_(x) -OR catalysts 300b and 304b constitute the NH₃ synthesizing catalysts 8 and the occlusive materials 11, respectively. To clarify, such NO_(x) -OR catalysts 300b and 304b are referred to as a NO_(x) occluding and NH₃ synthesizing (NO_(x) --NH₃) catalysts, hereinafter.

To perform the adsorbing and occluding process, when the first group 1a performs the rich operation and the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) --NH₃ catalyst 300b is made rich, the inflowing NO_(x) is converted to NH₃ on the NO_(x) --NH₃ catalyst 300b, and the NH₃ flows into and is adsorbed in the NH₃ -AO catalyst 302. Also, to perform the adsorbing and occluding process, when the second group 1b performs the lean operation and the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) --NH₃ catalyst 304b is made lean, some of the inflowing NO_(x) is partly occluded into the NO_(x) --NH₃ catalyst 304b, and the other flows into and is occluded in the NO_(x) -OR catalyst 306. Thus, the NO_(x) amount flowing into the NO_(x) -OR catalyst 306 is suppressed. Further, NO_(x) is prevented from flowing out from the NO_(x) -OR catalyst 306, in the adsorbing and occluding process.

To perform the desorbing, releasing, and purifying process, when the first group 1a performs the lean operation and the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) --NH₃ catalyst 300b is made lean, some of the inflowing NO_(x) is partly occluded into the NO_(x) --NH₃ catalyst 300b, and the other flows into the NH₃ -AO catalyst 302 and is reduced by the desorbed NH₃ from the NH₃ -AO catalyst 302. Also, to perform the desorbing, releasing, and purifying process, when the second group 1b performs the rich operation and the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) --NH₃ catalyst 304b is made rich, the inflowing NO_(x) is converted to NH₃ on the NO_(x) --NH₃ catalyst 304b. The NH₃ flows into the NO_(x) -OR catalyst 306, and reduces NO_(x) released from the NO_(x) -OR catalyst 306. Thus, the NO_(x) amount flowing into the NH₃ -AO catalyst 302 is suppressed. Further, NO_(x) is prevented from flowing out from the NH₃ -AO catalyst 302, in the desorbing, releasing, and purifying process.

Accordingly, the exhaust gas can be purified sufficiently by performing the adsorbing and occluding process, and the desorbing, releasing, and purifying process, alternately and repeatedly.

Note that, in the adsorbing and occluding process, the occluded NO_(x) is released from the NO_(x) --NH₃ catalyst 300b, because the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich. In the same manner, in the desorbing, releasing, and purifying process, the occluded NO_(x) is released from the NO_(x) --NH₃ catalyst 304b. It is considered that the released NO_(x) is reduced or converted to NH₃ on the NO_(x) --NH₃ catalyst 300b or the NO_(x) --NH₃ catalyst 304b.

On the other hand, if the adsorbing and occluding process is continued, that is, the first exhaust gas air-fuel ratio condition is continued, the occluded NO_(x) amount in the NO_(x) --NH₃ catalyst 304b becomes larger, and the NO_(x) occluding capacity becomes smaller. Therefore, in the first exhaust gas air-fuel ratio condition, the second group 1b temporarily performs the lean operation to release the occluded NO_(x) from the NO_(x) --NH₃ catalyst 304b. In the same manner, in the second exhaust gas air-fuel ratio condition, the first group 1a temporarily performs the lean operation to release the occluded NO_(x) from the NO_(x) --NH₃ catalyst 300b. This ensures the NO_(x) occluding capacity of the NO_(x) --NH₃ catalysts 300b and 304b, and preventing the NH₃ -AO catalyst 302 and the NO_(x) -OR catalyst 306 from flowing into a large amount of NO_(x).

However, if the second group 1b temporarily performs the rich operation in the first exhaust gas air-fuel ratio condition, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ purifying catalyst 16 is made rich, due to the rich operation of the first group 1a. Also, if the first group 1a temporarily performs the rich operation in the second exhaust gas air-fuel ratio condition, the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ purifying catalyst 16 is made rich, due to the rich operation of the second group 1b. Therefore, the secondary air supplying device 18 is provided to keep the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ purifying catalyst 16 lean, even when the occluded NO_(x) is to be released from the NO_(x) --NH₃ catalysts 300b and 304b.

In the above-mentioned embodiments, the upper and lower thresholds for the NO_(x) -OR catalysts UT(NO_(x)), UT1(NO_(x)), UT2(NO_(x)), LT(NO_(x)), LT1(NO_(x)), LT2(NO_(x)), the upper and lower thresholds for the NH₃ -AO catalysts UT(NH₃), UT1(NH₃), UT2(NH₃), LT(NH₃), LT1(NH₃), LT2(NH₃) may be determined in accordance with the characteristic, the component, or the volume of the corresponding catalyst, or the flow rate or the exhaust gas air-fuel ratio of the flowing exhaust gas into the corresponding catalyst, or the engine operating condition. The thresholds may be changed if required.

Further, the deterioration of the catalyst(s) located between the sensors 29 and 31 or between the sensors 30 and 32 may be detected on the basis of the output signals from the sensors 29 and 31 or those from the sensors 30 and 32. Namely, in the embodiment shown in FIG. 1, for example, the deterioration of the TW catalyst 8a can be detected on the basis of the output signals from the sensors 29 and 31. Or, in the embodiment shown in FIG. 39, the deterioration of the NO_(x) -OR catalyst 300b and the NH₃ -AO catalyst 302 can be detected on the basis of the output signals from the sensors 29 and 31, and that of the NO_(x) --NH₃ catalysts 304b and 306 can be detected on the basis of the output signals from the sensors 30 and 32.

Further, an air-fuel ratio sensor may be arranged in the exhaust passage close to the inlet or the outlet of the NH₃ purifying catalyst 16 for detecting the exhaust gas air-fuel ratio sensor of the exhaust gas flowing into the catalyst 16, and the secondary air supplying device may be controlled in accordance with the output signals from the sensor to keep the catalyst 16 under the oxidizing atmosphere.

Further, the rich air-fuel ratio (A/F)R with which the first or the second group 1a, 1b performs the rich operation, and the lean air-fuel ratio (A/F)L with which the first or the second group 1a, 1b performs the lean operation may be determined in accordance with the fuel consumption rate, the engine output torque, or the synthesized NH₃ amount, etc., in addition to the engine operating condition such as the engine load and the engine speed.

Finally, the first cylinder group 1a may be constructed from the plurality of the cylinders, and the second cylinder groups 1b may be constructed from the single cylinder, while the first group 1a is constructed from the single cylinder and the second group 1b is constructed from three cylinders, in the above-mentioned embodiments. However, lower fuel consumption rate is preferable, and thus the second group 1b, in which the lean operation is basically performed, is preferably constructed from the many cylinders as possible. Note that, when the first group 1a is constructed from the plurality of the cylinders, the target values (A/F)T for the engine air-fuel ratio of the cylinders are made identical to each other.

According to the present invention, it is possible to provide a method and a device for purifying an exhaust gas of an engine which can suppress the amount of NO_(x) flowing into the exhaust gas purifying catalyst with respect to that of NH₃, to thereby purify the exhaust gas sufficiently.

While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention. 

We claim:
 1. A method for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, the method comprising:making an exhaust gas air-fuel ratio of the exhaust gas of the first cylinder group rich, and introducing the exhaust gas to an NH₃ synthesizing catalyst to synthesize NH₃, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; introducing the exhaust gas of the first cylinder group including NH₃ and the exhaust gas of the second cylinder group including NO_(x), together, to an exhaust gas purifying catalyst; and controlling an amount of NO_(x) included in the exhaust gas of the second cylinder group and to be introduced to the exhaust gas purifying catalyst to prevent the NO_(x) amount from being larger than a NO_(x) amount which can be reduced by the NH₃ included in the exhaust gas of the first cylinder group and to be introduced to the exhaust gas purifying catalyst, wherein, on the exhaust gas purifying catalyst, the inflowing NO_(x) is reduced by the inflowing NH₃.
 2. A method according to claim 1, wherein the NO_(x) amount included in the exhaust gas of the second cylinder group and to be introduced to the exhaust gas purifying catalyst is controlled by contacting the exhaust gas with an occlusive material for occluding NO_(x) in the inflowing exhaust gas.
 3. A method according to claim 2, wherein the occlusive material comprises a NO_(x) occluding and reducing (NO_(x) -OR) catalyst, occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and wherein the method further comprises controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst to make the exhaust gas air-fuel ratio lean to occlude the inflowing NO_(x) partly in the NO_(x) -OR catalyst, and the exhaust gas air-fuel ratio rich to release the occluded NO_(x) from the NO_(x) -OR catalyst and reduce the NO_(x), alternately and repeatedly.
 4. A method according to claim 3, further comprising estimating an amount of NO_(x) occluded in the NO_(x) -OR catalyst, wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst is made lean and rich alternately and repeatedly, in accordance with the estimated occluded NO_(x) amount.
 5. A method according to claim 3, further comprising introducing the exhaust gas of the second cylinder group to a three way (TW) catalyst, prior to introducing the exhaust gas to the NO_(x) -OR catalyst.
 6. A method according to claim 3, wherein the second cylinder group has a plurality of cylinders divided into a plurality of cylinder subgroups, each subgroup being connected to the exhaust gas purifying catalyst via corresponding NO_(x) -OR catalysts, wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into each NO_(x) -OR catalyst is controlled to keep the exhaust gas air-fuel ratio of the exhaust gas flowing from the NO_(x) -OR catalysts to the exhaust gas purifying catalyst lean.
 7. A method according to claim 3, wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst is controlled by controlling the engine air-fuel ratio of the second cylinder group.
 8. A method according to claim 3, wherein the NO_(x) -OR catalyst includes: at least one substance selected from alkali metals such as potassium, sodium, lithium, and cesium, alkali earth metals including barium and calcium, rare earth metals including lanthanum and yttrium, and transition metals including iron and copper; and precious metals such as palladium, platinum, iridium, and rhodium.
 9. A method according to claim 1, wherein the NO_(x) amount included in the exhaust gas of the second cylinder group and to be introduced to the exhaust gas purifying catalyst is controlled by controlling the NO_(x) amount exhausted from the second cylinder group.
 10. A method according to claim 9, wherein the engine air-fuel ratio of the second cylinder group is controlled to control the exhausted NO_(x) amount.
 11. A method according to claim 9, wherein the number of the operating cylinder(s) in the second cylinder group is controlled to control the exhausted NO_(x) amount.
 12. A method according to claim 1, further comprising introducing the exhaust gas of the first and the second cylinder groups, together, to an adsorbent for adsorbing NH₃ in the inflowing exhaust gas therein, prior to or simultaneously introducing the exhaust gas to the exhaust gas purifying catalyst.
 13. A method according to claim 12, wherein the NH₃ synthesizing catalyst synthesizes almost no NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, wherein the adsorbent comprises an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and causing a reaction of NH₃ and NO_(x) in the NH₃ -AO catalyst to purify the NH₃ and the NO_(x) and to decrease an amount of NH₃ adsorbed in the NH₃ -AO catalyst when the inflowing exhaust gas includes NO_(x) therein and the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, wherein the method further comprises controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst to make the exhaust gas air-fuel ratio lean temporarily to stop feeding NH₃ to the NH₃ -AO catalyst, to thereby decrease the amount of NH₃ adsorbed in the NH₃ -AO catalyst.
 14. A method according to claim 13, further comprising estimating an amount of NH₃ adsorbed in the NH₃ -AO catalyst, wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is made lean in accordance with the estimated adsorbed NH₃ amount.
 15. A method according to claim 13, wherein the NH₃ -AO catalyst comprises a solid acid including zeolite, silica, silica-alumina, and titania, carrying a transition metal including copper, chrome, vanadium, titanium, iron, nickel, and one of cobalt and a precious metal including platinum, palladium, rhodium and iridium.
 16. A method according to claim 1, wherein the exhaust gas air-fuel ratio of the exhaust gas flowing the NH₃ synthesizing catalyst is controlled by controlling the engine air-fuel ratio of the first cylinder group.
 17. A method according to claim 1, wherein the exhaust gas purifying catalyst performs the purifying operation thereof when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and wherein the method further comprises keeping the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the exhaust gas purifying catalyst lean.
 18. A method according to claim 17, wherein the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the exhaust gas purifying catalyst is kept lean by supplying a secondary air into the exhaust gas flowing into the exhaust gas purifying catalyst.
 19. A method according to claim 1, wherein the NH₃ synthesizing catalyst is a three-way (TW) catalyst including at least one precious metal such as palladium, platinum, iridium, and rhodium.
 20. A method according to claim 1, wherein the exhaust gas purifying catalyst comprises a plurality of catalysts arranged, in series, in the interconnecting passage.
 21. A method according to claim 20, further comprising guiding the exhaust gas to be introduced into the exhaust gas purifying catalyst to force the exhaust gas to contact all the catalysts when the temperature of the inflowing exhaust gas is low, and to force the exhaust gas to bypass at least one of the catalysts when the temperature of the inflowing exhaust gas is high.
 22. A method according to claim 1, further comprising introducing the exhaust gas, exhausted from the exhaust gas purifying catalyst, to an NH₃ purifying catalyst for purifying NH₃ in the inflowing exhaust gas.
 23. A device for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, and first and second exhaust passage connected to the first and the second cylinder groups, respectively, the device comprising:an NH₃ synthesizing catalyst arranged in the first exhaust passage, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; an interconnecting exhaust passage interconnecting the first passage downstream of the NH₃ synthesizing catalyst and the second exhaust passage; an exhaust gas purifying catalyst arranged in the interconnecting passage for reducing the inflowing NO_(x) by the inflowing NH₃ ; first exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst; means for controlling the first exhaust gas air-fuel ratio control means to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst rich to synthesize NH₃ ; and NO_(x) amount control means for controlling an amount of NO_(x) flowing from the second exhaust passage into the exhaust gas purifying catalyst to prevent the NO_(x) amount from being larger than a NO_(x) amount which can be reduced by the NH₃ flowing from the first exhaust passage into the exhaust gas purifying catalyst, wherein, on the exhaust gas purifying catalyst, the inflowing NO_(x) is reduced by the inflowing NH₃.
 24. A device according to claim 23, wherein the NO_(x) amount control means comprises an occlusive material arranged in the second exhaust passage for occluding NO_(x) in the inflowing exhaust gas.
 25. A device according to claim 24, wherein the occlusive material comprises a NO_(x) occluding and reducing (NO_(x) -OR) catalyst, occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and wherein the device further comprises second exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst, and NO_(x) occlusion control means for controlling the second exhaust gas air-fuel ratio control means to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst lean to occlude the inflowing NO_(x) partly in the NO_(x) -OR catalyst, and the exhaust gas air-fuel ratio rich to release the occluded NO_(x) from the NO_(x) -OR catalyst and reduce the NO_(x), alternately and repeatedly.
 26. A device according to claim 25, further comprising occluded NO_(x) amount estimating means for estimating an amount of NO_(x) occluded in the NO_(x) -OR catalyst, wherein the NO_(x) occlusion control means makes the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst lean and rich alternately and repeatedly, in accordance with the estimated occluded NO_(x) amount.
 27. A device according to claim 25, further comprising a three way (TW) catalyst arranged in the second exhaust passage upstream of the NO_(x) -OR catalyst.
 28. A device according to claim 25, wherein the second cylinder group has a plurality of the cylinders divided into a plurality of cylinder subgroups, each subgroup being connected to the exhaust gas purifying catalyst via corresponding NO_(x) -OR catalysts, wherein the NO_(x) occlusion control means controls the exhaust gas air-fuel ratio of the exhaust gas flowing into each NO_(x) -OR catalyst to keep the exhaust gas air-fuel ratio of the exhaust gas flowing from the second passage to the exhaust gas purifying catalyst lean.
 29. A device according to claim 25, wherein the second exhaust gas air-fuel ratio control means controls the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_(x) -OR catalyst by controlling the engine air-fuel ratio of the second cylinder group.
 30. A device according to claim 25, wherein the NO_(x) -OR catalyst includes: at least one substance selected from alkali metals such as potassium, sodium, lithium, and cesium, alkali earth metals including barium and calcium, rare earth metals including lanthanum and yttrium, and transition metals including iron and copper; and precious metals including palladium, platinum, iridium, and rhodium.
 31. A device according to claim 23, wherein the NO_(x) amount control means comprises an exhausted NO_(x) amount control means for controlling a NO_(x) amount exhausted from the second cylinder group into the second exhaust passage, the NO_(x) amount control means controlling the NO_(x) amount flowing from the second exhaust passage into the exhaust gas purifying catalyst by controlling the NO_(x) amount exhausted from the second cylinder group.
 32. A device according to claim 31, wherein the exhausted NO_(x) amount control means controls the engine air-fuel ratio of the second cylinder group to thereby control the exhausted NO_(x) amount.
 33. A device according to claim 31, wherein the exhausted NO_(x) amount control means controls the number of the operating cylinder(s) in the second cylinder group to thereby control the exhausted NO_(x) amount.
 34. A device according to claim 23, further comprising an adsorbent arranged in the interconnecting passage in or downstream of the exhaust gas purifying catalyst for adsorbing NH₃ in the inflowing exhaust gas therein.
 35. A device according to claim 34, wherein the NH₃ synthesizing catalyst synthesizes almost no NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, wherein the adsorbent comprises an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and causing a reaction of NH₃ and NO_(x) in the NH₃ -AO catalyst to purify the NH₃ and the NO_(x) and to decrease an amount of NH₃ adsorbed in the NH₃ -AO catalyst when the inflowing exhaust gas includes NO_(x) therein and the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, wherein the device further comprises means for controlling the first exhaust gas air-fuel ratio control means to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst lean temporarily to stop feeding NH₃ to the NH₃ -AO catalyst, to thereby decrease the amount of NH₃ adsorbed in the NH₃ -AO catalyst.
 36. A device according to claim 35, further comprising adsorbed NH₃ amount estimating means for estimating an amount of NH₃ adsorbed in the NH₃ -AO catalyst, wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is made lean in accordance with the estimated adsorbed NH₃ amount.
 37. A device according to claim 35, wherein the NH₃ -AO catalyst comprises a solid acid including zeolite, silica, silica-alumina, and titania, carrying a transition metal including copper, chrome, vanadium, titanium, iron, nickel, and cobalt or a precious metal including platinum, palladium, rhodium and iridium.
 38. A device according to claim 23, wherein the first exhaust gas air-fuel ratio control means controls the exhaust gas air-fuel ratio of the exhaust gas passing the NH₃ synthesizing catalyst by controlling the engine air-fuel ratio of the first cylinder group.
 39. A device according to claim 23, wherein the exhaust gas purifying catalyst performs the purifying operation thereof when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and wherein the device further comprises keeping-lean means for keeping the exhaust gas air-fuel ratio of the exhaust gas mixture flowing into the exhaust gas purifying catalyst lean.
 40. A device according to claim 39, wherein the keeping-lean means comprises a secondary air supplying means arranged in the interconnecting passage upstream of the exhaust gas purifying catalyst for supplying a secondary air into the exhaust gas flowing into the exhaust gas purifying catalyst.
 41. A device according to claim 23, wherein the NH₃ synthesizing catalyst is a three-way (TW) catalyst including at least one precious metal such as palladium, platinum, iridium, and rhodium.
 42. A device according to claim 23, wherein the exhaust gas purifying catalyst comprises a plurality of catalysts arranged, in series, in the interconnecting passage.
 43. A device according to claim 42, further comprising guide means for guiding the exhaust gas flowing through the interconnecting passage to force the exhaust gas to contact all of the catalysts when the temperature of the inflowing exhaust gas is low, and to force the exhaust gas to bypass at least one of the catalysts when the temperature of the inflowing exhaust gas is high.
 44. A device according to claim 23, further comprising an NH₃ purifying catalyst arranged in the interconnecting passage downstream of the exhaust gas purifying catalyst for purifying NH₃ in the inflowing exhaust gas.
 45. A method for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, the method comprising:making the exhaust gas air-fuel ratio of the exhaust gas of the first cylinder group rich, and introducing the exhaust gas to an NH₃ synthesizing catalyst to synthesize NH₃, to form the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; making the exhaust gas air-fuel ratio of the exhaust gas of the second cylinder group lean, to form the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean; performing a first introducing condition where the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich is introduced to an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst and the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean is introduced to a NO_(x) occluding and reducing (NO_(x) -OR) catalyst, the NH₃ -AO catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and causing a reaction of NH₃ and NO_(x) in the NH₃ -AO catalyst to purify the NH₃ and the NO_(x) and to decrease an amount of NH₃ adsorbed in the NH₃ -AO catalyst when the inflowing exhaust gas includes NO_(x) therein and the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, the NO_(x) -OR catalyst occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; performing a second introducing condition where the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich is introduced to the NO_(x) -OR catalyst and the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean is introduced to the NH₃ -AO catalyst; and performing the first and the second introducing conditions alternately and repeatedly.
 46. A method according to claim 45, further comprising estimating an amount of NO_(x) occluded in the NO_(x) -OR catalyst, wherein the introducing conditions are changed in accordance with the estimated occluded NO_(x) amount.
 47. A method according to claim 45, further comprising estimating an amount of NH₃ adsorbed in the NH₃ -AO catalyst, wherein the introducing conditions are changed in accordance with the estimated adsorbed NH₃ amount.
 48. A method according to claim 45, further comprising suppressing an amount of NO_(x) in the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean.
 49. A method according to claim 48, wherein the NO_(x) amount in the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean is suppressed by introducing the exhaust gas to an additional NO_(x) occluding and reducing (NO_(x) -OR) catalyst.
 50. A method according to claim 49, further comprising controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the additional NO_(x) -OR catalyst to make the exhaust gas air-fuel ratio rich temporarily to release the occluded NO_(x) from the additional NO_(x) -OR catalyst.
 51. A method according to claim 45, wherein the engine is provided with: a first NH₃ introducing passage being able to introduce the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich to the NH₃ -AO catalyst; a second NH₃ introducing passage being able to introduce the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich to the NO_(x) -OR catalyst; a first NO_(x) introducing passage being able to introduce the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean to the NO_(x) -OR catalyst; a second NO_(x) introducing passage being able to introduce the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean to the NH₃ -AO catalyst; an NH₃ switching valve selectively introducing the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich to one of the first and the second NH₃ introducing passages; and a NO_(x) switching valve selectively introducing the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean to one of the first and the second NO_(x) introducing passages, and wherein the NH₃ and NO_(x) switching valves are controlled to introduce the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich to the first NH₃ introducing passage and introduce the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean to the first NO_(x) introducing passage when the first introducing condition is to be performed, and to introduce the exhaust gas including NH₃ of which the exhaust gas air-fuel ratio is rich to the second NH₃ introducing passage and introduce the exhaust gas including NO_(x) of which the exhaust gas air-fuel ratio is lean to the second NO_(x) introducing passage when the second introducing condition is to be performed.
 52. A method according to claim 45, wherein the NH₃ synthesizing catalyst is a three-way (TW) catalyst including at least one precious metal such as palladium, platinum, iridium, and rhodium.
 53. A method according to claim 45, wherein the NH₃ -AO catalyst comprises a solid acid including zeolite, silica, silica-alumina, and titania, carrying a transition metal including copper, chrome, vanadium, titanium, iron, nickel, and cobalt or a precious metal including platinum, palladium, rhodium and iridium.
 54. A method according to claim 45, wherein the NO_(x) -OR catalyst includes: at least one substance selected from alkali metals including potassium, sodium, lithium, and cesium, alkali earth metals including barium and calcium, rare earth metals including lanthanum and yttrium, and transition metals including iron and copper; and precious metals including palladium, platinum, iridium, and rhodium.
 55. A method according to claim 45, further comprising introducing at least one of the exhaust gas discharged from the NH₃ -AO catalyst and that from the NO_(x) -OR catalyst to an NH₃ purifying catalyst for purifying NH₃ in the inflowing exhaust gas.
 56. A device for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, and first and second exhaust passage connected to the first and the second cylinder groups, respectively, the device comprising:an NH₃ synthesizing catalyst arranged in the first exhaust passage, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst selectively connected to one of the first exhaust passage downstream of the NH₃ synthesizing catalyst and the second exhaust passage, the NH₃ -AO catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and causing a reaction of NH₃ and NO_(x) in the NH₃ -AO catalyst to purify the NH₃ and the NO_(x) and to decrease an amount of NH₃ adsorbed in the NH₃ -AO catalyst when the inflowing exhaust gas includes NO_(x) therein and the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean; a NO_(x) occluding and reducing (NO_(x) -OR) catalyst selectively connected to one of the first exhaust passage downstream of the NH₃ synthesizing catalyst and the second exhaust passage, the NO_(x) -OR catalyst occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; first exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst; second exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing through the second exhaust passage; means for controlling the first exhaust gas air-fuel ratio control means to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst rich to synthesize NH₃ ; means for controlling the second exhaust gas air-fuel ratio control means to make the exhaust gas air-fuel ratio of the exhaust gas flowing through the second exhaust passage lean; first connecting condition performing means for performing a first connecting condition where the first exhaust passage downstream of the NH₃ synthesizing catalyst is connected to the NH₃ -AO catalyst and the second exhaust passage is connected to the NO_(x) -OR catalyst; second connecting condition performing means for performing a second connecting condition where the first exhaust passage downstream of the NH₃ synthesizing catalyst is connected to the NO_(x) -OR catalyst and the second exhaust passage is connected to the NH₃ -AO catalyst; and connecting condition control means for controlling the first and the second connecting condition performing means to perform the first and the second connecting conditions alternately and repeatedly.
 57. A device according to claim 56, further comprising occluded NO_(x) amount estimating means for estimating an amount of NO_(x) occluded in the NO_(x) -OR catalyst, wherein the connecting conditions are changed in accordance with the estimated occluded NO_(x) amount.
 58. A device according to claim 56, further comprising adsorbed NH₃ amount estimating means for estimating an amount of NH₃ adsorbed in the NH₃ -AO catalyst, wherein the connecting conditions are changed in accordance with the estimated adsorbed NH₃ amount.
 59. A device according to claim 56, further comprising suppressing means for suppressing an NO_(x) amount exhausted from the second exhaust passage.
 60. A device according to claim 59, wherein the suppressing means comprises an additional NO_(x) occluding and reducing (NO_(x) -OR) catalyst arranged in the second exhaust passage.
 61. A device according to claim 60, further comprising means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the additional NO_(x) -OR catalyst to make the exhaust gas air-fuel ratio rich temporarily to release the occluded NO_(x) from the additional NO_(x) -OR catalyst.
 62. A device according to claim 56, further comprising: a first NH₃ introducing passage being able to connect the first exhaust passage downstream of the NH₃ synthesizing catalyst to the NH₃ -AO catalyst; a second NH₃ introducing passage being able to connect the first exhaust passage downstream of the NH₃ synthesizing catalyst to the NO_(x) -OR catalyst; a first NO_(x) introducing passage being able to connect the second exhaust passage to the NO_(x) -OR catalyst; a second NO_(x) introducing passage being able to connect the second exhaust passage to the NH₃ -AO catalyst; an NH₃ switching valve selectively connecting the first exhaust passage downstream of the NH₃ synthesizing catalyst to one of the first and the second NH₃ introducing passages; and a NO_(x) switching valve selectively connecting the second exhaust passage to one of the first and the second NO_(x) introducing passages, wherein the connecting condition control means controls the NH₃ and NO_(x) switching valves to connect the first exhaust passage downstream of the NH₃ synthesizing catalyst to the first NH₃ introducing passage and connect the second exhaust passage to the first NO_(x) introducing passage when the first connecting condition is to be performed, and to connect the first exhaust passage downstream of the NH₃ synthesizing catalyst to the second NH₃ introducing passage and connect the second exhaust passage to the second NO_(x) introducing passage when the second connecting condition is to be performed.
 63. A device according to claim 56, wherein the NH₃ synthesizing catalyst is a three-way (TW) catalyst including at least one precious metal such as palladium, platinum, iridium, and rhodium.
 64. A device according to claim 56, wherein the NH₃ -AO catalyst comprises a solid acid including zeolite, silica, silica-alumina, and titania, carrying a transition metal including copper, chrome, vanadium, titanium, iron, nickel, and cobalt or a precious metal including platinum, palladium, rhodium and iridium.
 65. A device according to claim 56, wherein the NO_(x) -OR catalyst includes: at least one substance selected from alkali metals including potassium, sodium, lithium, and cesium, alkali earth metals including barium and calcium, rare earth metals including lanthanum and yttrium, and transition metals including iron and copper; and precious metals including palladium, platinum, iridium, and rhodium.
 66. A device according to claim 56, further comprising an NH₃ purifying catalyst arranged downstream of at least one of the NH₃ -AO catalyst and the NO_(x) -OR catalyst for purifying NH₃ in the inflowing exhaust gas.
 67. A method for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, the method comprising:introducing the exhaust gas of the first cylinder group to a first NH₃ synthesizing catalyst and an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst, in turn, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and synthesizing almost no NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and the NH₃ -AO catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and causing a reaction of NH₃ and NO_(x) in the NH₃ -AO catalyst to purify the NH₃ and the NO_(x) and to decrease an amount of NH₃ adsorbed in the NH₃ -AO catalyst when the inflowing exhaust gas includes NO_(x) therein and the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean; introducing the exhaust gas of the second cylinder group to a second NH₃ synthesizing catalyst and a NO_(x) occluding and reducing (NO_(x) -OR) catalyst, in turn, the NO_(x),OR catalyst occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; performing a first exhaust gas air-fuel ratio condition where the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst is made rich, and that of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst is made lean; performing a second exhaust gas air-fuel ratio condition where the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst is made lean, and that of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst is made rich; and performing the first and the second exhaust gas air-fuel ratio conditions alternately and repeatedly.
 68. A method according to claim 67, further comprising estimating an amount of NO_(x) occluded in the NO_(x) -OR catalyst, wherein the exhaust gas air-fuel ratio conditions are changed in accordance with the estimated occluded NO_(x) amount.
 69. A method according to claim 67, further comprising estimating an amount of NH₃ adsorbed in the NH₃ -AO catalyst, wherein the exhaust gas air-fuel ratio conditions are changed in accordance with the estimated adsorbed NH₃ amount.
 70. A method according to claim 67, wherein the engine air-fuel ratio of the first cylinder group is controlled to control the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst.
 71. A method according to claim 67, wherein the engine air-fuel ratio of the second cylinder group is controlled to control the exhaust gas air-fuel ratio of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst.
 72. A method according to claim 67, wherein the NH₃ synthesizing catalyst is a three-way (TW) catalyst including at least one precious metal such as palladium, platinum, iridium, and rhodium.
 73. A method according to claim 67, wherein the NH₃ synthesizing catalyst is a NO_(x) -OR catalyst including: at least one substance selected from alkali metals including potassium, sodium, lithium, and cesium, alkali earth metals including barium and calcium, rare earth metals including lanthanum and yttrium, and transition metals such as iron and copper; and precious metals including palladium, platinum, iridium, and rhodium.
 74. A method according to claim 67, wherein the NH₃ -AO catalyst comprises a solid acid including zeolite, silica, silica-alumina, and titania, carrying a transition metal including copper, chrome, vanadium, titanium, iron, nickel, and cobalt or a precious metal including platinum, palladium, rhodium and iridium.
 75. A method according to claim 67, wherein the NO_(x) -OR catalyst includes: at least one substance selected from alkali metals including potassium, sodium, lithium, and cesium, alkali earth metals including barium and calcium, rare earth metals including lanthanum and yttrium, and transition metals including iron and copper; and precious metals including palladium, platinum, iridium, and rhodium.
 76. A method according to claim 67, further comprising introducing at least one of the exhaust gas discharged from the NH₃ -AO catalyst and that from the NO_(x) -OR catalyst to an NH₃ purifying catalyst for purifying NH₃ in the inflowing exhaust gas.
 77. A device for purifying an exhaust gas of an engine having a plurality of cylinders divided into first and second cylinder groups, and first and second exhaust passage connected to the first and the second cylinder groups, respectively, the device comprising:a first NH₃ synthesizing catalyst arranged in the first exhaust passage and a second NH₃ synthesizing catalyst arranged in the second exhaust passage, each NH₃ synthesizing catalyst synthesizing NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and synthesizing almost no NH₃ from NO_(x) in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean; an NH₃ adsorbing and oxidizing (NH₃ -AO) catalyst arranged in the first exhaust passage downstream of the first NH₃ synthesizing catalyst, the NH₃ -AO catalyst adsorbing NH₃ in the inflowing exhaust gas therein, and causing a reaction of NH₃ and NO_(x) in the NH₃ -AO catalyst to purify the NH₃ and the NO_(x) and to decrease an amount of NH₃ adsorbed in the NH₃ -AO catalyst when the inflowing exhaust gas includes NO_(x) therein and the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean; a NO_(x) occluding and reducing (NO_(x) -OR) catalyst arranged in the second exhaust passage downstream of the second NH₃ synthesizing catalyst, the NO_(x) -OR catalyst occluding NO_(x) in the inflowing exhaust gas therein when the exhaust gas air-fuel ratio of the inflowing exhaust gas is lean, and releasing the occluded NO_(x) therefrom and reducing the NO_(x) when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; a first exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst; a second exhaust gas air-fuel ratio control means for controlling the exhaust gas air-fuel ratio of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst; first exhaust gas air-fuel ratio condition performing means for controlling the first and the second exhaust gas air-fuel ratio control means to perform a first exhaust gas air-fuel ratio condition where the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst is made rich, and that of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst is made lean; second exhaust gas air-fuel ratio condition performing means for controlling the first and the second exhaust gas air-fuel ratio control means to perform a second exhaust gas air-fuel ratio condition where the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst is made lean, and that of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst is made rich; and exhaust gas air-fuel ratio condition control means for controlling the first and the second exhaust gas air-fuel ratio condition performing means to perform the first and the second exhaust gas air-fuel ratio conditions alternately and repeatedly.
 78. A device according to claim 77, further comprising occluded NO_(x) amount estimating means for estimating an amount of NO_(x) occluded in the NO_(x) -OR catalyst, wherein the exhaust gas air-fuel ratio conditions are changed in accordance with the estimated occluded NO_(x) amount.
 79. A device according to claim 77, further comprising adsorbed NH₃ amount estimating means for estimating an amount of NH₃ adsorbed in the NH₃ -AO catalyst, wherein the exhaust gas air-fuel ratio conditions are changed in accordance with the estimated adsorbed NH₃ amount.
 80. A device according to claim 77, wherein the first exhaust gas air-fuel ratio control means controls the engine air-fuel ratio of the first cylinder group to control the exhaust gas air-fuel ratio of the exhaust gas flowing into the first NH₃ synthesizing catalyst and the NH₃ -AO catalyst.
 81. A device according to claim 77, wherein the second exhaust gas air-fuel ratio control means controls the engine air-fuel ratio of the second cylinder group to control the exhaust gas air-fuel ratio of the exhaust gas flowing into the second NH₃ synthesizing catalyst and the NO_(x) -OR catalyst.
 82. A device according to claim 77, wherein the NH₃ synthesizing catalyst is a three-way (TW) catalyst including at least one precious metal such as palladium, platinum, iridium, and rhodium.
 83. A device according to claim 77, wherein the NH₃ synthesizing catalyst is a NO_(x) -OR catalyst including: at least one substance selected from alkali metals including potassium, sodium, lithium, and cesium, alkali earth metals including barium and calcium, rare earth metals including lanthanum and yttrium, and transition metals including iron and copper; and precious metals including palladium, platinum, iridium, and rhodium.
 84. A device according to claim 77, wherein the NH₃ -AO catalyst comprises a solid acid including zeolite, silica, silica-alumina, and titania, carrying a transition metal including copper, chrome, vanadium, titanium, iron, nickel, and cobalt or a precious metal including platinum, palladium, rhodium and iridium.
 85. A device according to claim 77, wherein the NO_(x) -OR catalyst includes: at least one substance selected from alkali metals including potassium, sodium, lithium, and cesium, alkali earth metals including barium and calcium, rare earth metals including lanthanum and yttrium, and transition metals including iron and copper; and precious metals including palladium, platinum, iridium, and rhodium.
 86. A device according to claim 77, further comprising an NH₃ purifying catalyst arranged downstream of at least one of the NH₃ -AO catalyst and the NO_(x) -OR catalyst for purifying NH₃ in the inflowing exhaust gas. 